Anatomy Quiz: Filtration Reabsorption And Secretion
16 questions · exam conditions
0:00
Filtration Reabsorption And SecretionQuestion 1 of 16

A patient with diabetes mellitus has a plasma glucose concentration of 300 mg/dL (normal: 70-100 mg/dL). If the renal threshold for glucose is 180 mg/dL and the glomerular filtration rate is 120 mL/min, what is the primary reason glucose appears in this patient's urine?

Glomerular filtration is impaired, preventing normal glucose handling
Tubular reabsorption capacity for glucose has been exceeded by the filtered load
Active secretion of glucose into the tubular lumen is occurring abnormally
The glomerular filtration barrier has become permeable to large glucose molecules
Decreased blood flow to the kidneys is causing glucose retention in the filtrate
← Back to quizzes

Anatomy Quiz

Anatomy Quiz: Filtration Reabsorption And Secretion

Practice Filtration Reabsorption And Secretion in Anatomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Filtration Reabsorption And Secretion, giving you a quick way to practice the rules, question types, and explanations that matter most for Anatomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A patient with diabetes mellitus has a plasma glucose concentration of 300 mg/dL (normal: 70-100 mg/dL). If the renal threshold for glucose is 180 mg/dL and the glomerular filtration rate is 120 mL/min, what is the primary reason glucose appears in this patient's urine?

  1. Glomerular filtration is impaired, preventing normal glucose handling
  2. Tubular reabsorption capacity for glucose has been exceeded by the filtered load (correct answer)
  3. Active secretion of glucose into the tubular lumen is occurring abnormally
  4. The glomerular filtration barrier has become permeable to large glucose molecules
  5. Decreased blood flow to the kidneys is causing glucose retention in the filtrate
Explanation: When you encounter questions about glucose in urine (glucosuria), think about the three-step process of kidney function: filtration, reabsorption, and secretion. Normal glucose handling depends on the balance between how much glucose is filtered and how much the tubules can reabsorb. In this case, the patient's plasma glucose (300 mg/dL) far exceeds the renal threshold of 180 mg/dL. The renal threshold represents the plasma concentration at which the tubular transport proteins become saturated and can no longer reabsorb all the filtered glucose. With a GFR of 120 mL/min, this patient is filtering an enormous glucose load that overwhelms the sodium-glucose cotransporters (SGLT1 and SGLT2) in the proximal tubule. The excess glucose that cannot be reabsorbed spills into the urine. Option A is incorrect because the GFR of 120 mL/min is actually normal, indicating proper glomerular function. Option C misunderstands glucose handling—kidneys don't actively secrete glucose; they only filter and reabsorb it. Option D shows confusion about molecular size—glucose is a small molecule that's normally filtered freely at the glomerulus, regardless of diabetes status. The key concept here is transport maximum (Tm)—the maximum rate at which tubular cells can transport a substance. Once you exceed this capacity, the excess appears in urine. Study tip: For anatomy and physiology exams, always remember that glucosuria in diabetes results from filtered load exceeding reabsorption capacity, not from kidney damage (unless diabetic nephropathy develops later).

Question 2

A patient is given para-aminohippuric acid (PAH) to measure renal plasma flow. If 90% of PAH is removed from the plasma in a single pass through the kidneys, and the patient's hematocrit is 45%, which statement best explains how PAH clearance occurs?

  1. PAH is freely filtered at the glomerulus and undergoes no further tubular processing
  2. PAH is both freely filtered at the glomerulus and actively secreted by proximal tubule cells (correct answer)
  3. PAH is actively reabsorbed in the proximal tubule after being freely filtered
  4. PAH is secreted into the tubules without being filtered at the glomerulus
  5. PAH clearance occurs primarily through passive diffusion across all nephron segments
Explanation: When you encounter questions about renal clearance measurements, focus on understanding how different substances move through the nephron. Para-aminohippuric acid (PAH) is specifically used to measure renal plasma flow because of its unique handling by the kidneys. The key insight is that 90% clearance in a single pass indicates PAH undergoes both filtration and secretion. If PAH were only filtered at the glomerulus, the clearance would be much lower (around 20% of the plasma volume), since filtration alone removes only the portion of plasma that becomes filtrate. The high 90% clearance means additional PAH must be actively removed from the remaining plasma flowing through the peritubular capillaries. Option B correctly identifies that PAH is both freely filtered at the glomerulus and actively secreted by proximal tubule cells. This dual mechanism explains the high extraction ratio. Option A is wrong because filtration alone cannot achieve 90% clearance - this would describe inulin, which has much lower clearance. Option C incorrectly suggests reabsorption, which would decrease rather than increase PAH removal from plasma. Option D is incorrect because PAH does undergo glomerular filtration in addition to secretion - it's not secretion-only. The 45% hematocrit is included to test whether you understand that clearance measures plasma flow, not whole blood flow, but the main concept being tested is the mechanism of PAH clearance. Remember: High clearance values for any substance suggest active secretion is occurring beyond simple filtration. PAH's nearly complete extraction makes it ideal for measuring total renal plasma flow.

Question 3

During severe dehydration, the thick ascending limb of the loop of Henle continues to transport sodium chloride out of the tubular fluid while remaining impermeable to water. What is the primary functional significance of this process in the dehydrated state?

  1. It directly increases water reabsorption in the ascending limb to conserve body fluids
  2. It maintains the medullary concentration gradient necessary for concentrating urine distally (correct answer)
  3. It increases sodium excretion to maintain electrolyte balance during fluid loss
  4. It decreases the osmolarity of tubular fluid to reduce further water loss
  5. It prevents excessive potassium loss that would occur with continued water reabsorption
Explanation: When you encounter questions about nephron function during dehydration, focus on how different segments work together to concentrate urine and conserve water. The thick ascending limb of the loop of Henle is crucial for creating the concentration gradient that allows your kidneys to produce concentrated urine. This segment actively transports sodium and chloride out of the tubular fluid while being completely impermeable to water. During severe dehydration, this process becomes even more critical because it maintains the high osmolarity in the medullary interstitium that's essential for water reabsorption downstream in the collecting duct. Option B is correct because the sodium chloride transport creates and maintains the medullary concentration gradient. Without this gradient, the collecting duct couldn't reabsorb water effectively, even with maximum ADH stimulation. Option A is wrong because the thick ascending limb itself never reabsorbs water—it's always impermeable to water. Option C misses the point entirely; during dehydration, you want to retain sodium, not excrete more of it. The transported sodium doesn't leave the kidney—it accumulates in the medullary interstitium. Option D is backwards; while the ascending limb does dilute the tubular fluid, this isn't to reduce water loss but to create the concentration gradient needed for water conservation elsewhere. Remember: the loop of Henle functions as a countercurrent multiplier system. The ascending limb's job is gradient creation, while the descending limb and collecting duct handle the actual water reabsorption using that gradient.

Question 4

A patient is administered inulin and para-aminohippuric acid (PAH) simultaneously to assess kidney function. Laboratory results show an inulin clearance of 90 mL/min and a PAH clearance of 540 mL/min. If the patient's hematocrit is 40%, what is the filtration fraction?

  1. 0.10 (10%)
  2. 0.17 (17%) (correct answer)
  3. 0.20 (20%)
  4. 0.25 (25%)
  5. 0.60 (60%)
Explanation: When you see questions involving inulin and PAH clearances, you're dealing with kidney function assessment. These substances help measure glomerular filtration rate (GFR) and renal plasma flow (RPF), which together determine the filtration fraction. To find the filtration fraction, you need to calculate the ratio of GFR to RPF. Inulin clearance directly measures GFR because inulin is freely filtered but neither reabsorbed nor secreted. Here, GFR = 90 mL/min. PAH clearance measures renal plasma flow because PAH is both filtered and actively secreted, meaning virtually all PAH is cleared from plasma in one pass through the kidneys. So RPF = 540 mL/min. The filtration fraction formula is: Filtration Fraction=GFRRPF=90540=0.167\text{Filtration Fraction} = \frac{\text{GFR}}{\text{RPF}} = \frac{90}{540} = 0.167 or approximately 17%. Choice A (10%) represents a calculation error, possibly from incorrectly using the hematocrit value in the denominator. Choice C (20%) might result from rounding 0.167 up too aggressively or making an arithmetic mistake. Choice D (25%) could come from confusing the relationship between the clearance values or incorrectly incorporating the hematocrit of 40% into the calculation. Note that the hematocrit value is irrelevant for this calculation—it's included as a distractor. The filtration fraction depends only on the ratio of what's filtered (inulin clearance) to what reaches the kidney (PAH clearance). Remember: Filtration fraction questions always use the simple ratio GFR/RPF, typically around 15-20% in healthy individuals.

Question 5

A 45-year-old patient presents with edema and proteinuria. Laboratory analysis reveals damaged podocytes in the glomerular filtration barrier. The patient's 24-hour urine collection shows significant protein loss, but creatinine clearance remains within normal limits.

Based on this clinical presentation, which aspect of glomerular filtration has been primarily compromised?

  1. The size-selective properties of the filtration barrier, allowing larger molecules through
  2. The charge-selective properties of the filtration barrier, reducing albumin repulsion (correct answer)
  3. The total surface area available for filtration, decreasing overall filtration rate
  4. The hydrostatic pressure gradient across the glomerular capillaries
  5. The oncotic pressure gradient that normally opposes filtration
Explanation: When you encounter questions about glomerular filtration and proteinuria, focus on the three-layered filtration barrier and its dual selective properties: size and charge selectivity. The correct answer is B because damaged podocytes specifically compromise the charge-selective properties of the filtration barrier. Podocytes contain negatively charged glycoproteins that normally repel albumin and other negatively charged proteins, preventing their filtration. When podocytes are damaged, this charge repulsion is lost, allowing albumin to pass through despite being the right size to normally be retained. This explains why the patient has significant proteinuria while maintaining normal creatinine clearance—the barrier can still filter appropriately sized molecules but has lost its ability to repel proteins based on charge. Answer A is incorrect because size selectivity involves the physical pore size of the filtration barrier, which would affect creatinine clearance if compromised. Since creatinine clearance is normal, size selectivity remains intact. Answer C is wrong because reduced filtration surface area would decrease overall filtration rate and creatinine clearance, but the patient's creatinine clearance is normal. Answer D is incorrect because altered hydrostatic pressure would affect the overall filtration rate and creatinine clearance, not selectively allow protein passage while maintaining normal clearance. Study tip: Remember that podocyte damage specifically affects charge selectivity, leading to proteinuria with preserved creatinine clearance. This pattern—protein loss without reduced filtration rate—is the hallmark of charge-selective barrier dysfunction.

Question 6

A patient with chronic kidney disease has a creatinine clearance of 30 mL/min (normal: 120 mL/min). If this patient's plasma creatinine concentration is 4.0 mg/dL and urine creatinine concentration is 40 mg/dL, what is the most likely explanation for these findings?

  1. Normal glomerular filtration with increased tubular reabsorption of creatinine
  2. Decreased glomerular filtration with normal tubular handling of creatinine (correct answer)
  3. Normal glomerular filtration with increased tubular secretion of creatinine
  4. Increased glomerular filtration with decreased tubular secretion of creatinine
  5. Decreased glomerular filtration with increased tubular reabsorption of creatinine
Explanation: When you encounter kidney function questions, focus on the relationship between creatinine clearance, plasma creatinine, and glomerular filtration rate (GFR). Creatinine clearance directly reflects GFR because creatinine is filtered by the glomeruli but neither reabsorbed nor secreted significantly by healthy tubules. Let's verify the math: Creatinine clearance = (Urine creatinine × Urine flow rate) ÷ Plasma creatinine. The given clearance of 30 mL/min is dramatically reduced from the normal 120 mL/min, indicating severely impaired glomerular filtration. The elevated plasma creatinine (4.0 mg/dL vs. normal ~1.0 mg/dL) confirms this - when filtration drops, creatinine accumulates in the blood. Option B correctly identifies decreased glomerular filtration with normal tubular handling. The math works out because even though filtration is impaired, the tubules are still handling creatinine normally (no significant reabsorption or secretion). Option A is wrong because normal glomerular filtration would maintain normal plasma creatinine levels, and increased tubular reabsorption would further elevate plasma creatinine beyond what we see. Option C is incorrect because normal glomerular filtration wouldn't produce this elevated plasma creatinine, and increased tubular secretion would actually lower plasma creatinine. Option D makes no sense - increased glomerular filtration would lower plasma creatinine, not raise it to 4.0 mg/dL. Remember: In chronic kidney disease, the primary problem is loss of functioning nephrons, which directly reduces GFR. Elevated plasma creatinine is your key indicator of impaired filtration function.

Question 7

In the distal convoluted tubule and collecting duct, aldosterone enhances sodium reabsorption. If a patient has primary aldosteronism (excessive aldosterone production), which combination of urinary changes would be expected?

  1. Increased sodium excretion and decreased potassium excretion in the urine
  2. Decreased sodium excretion and increased potassium excretion in the urine (correct answer)
  3. Increased sodium excretion and increased potassium excretion in the urine
  4. Decreased sodium excretion and decreased potassium excretion in the urine
  5. Normal sodium excretion with decreased potassium excretion in the urine
Explanation: Questions about hormone effects on kidney function test your understanding of how specific hormones regulate electrolyte balance. When you encounter aldosterone scenarios, focus on its primary action: enhancing sodium reabsorption in exchange for potassium secretion. Aldosterone works by binding to mineralocorticoid receptors in the distal convoluted tubule and collecting duct, stimulating the production of epithelial sodium channels (ENaC) and sodium-potassium pumps. This increases sodium reabsorption from the urine back into the bloodstream. However, this process is coupled with potassium secretion—as more sodium is reabsorbed, more potassium is actively secreted into the urine to maintain electrochemical balance. In primary aldosteronism, excessive aldosterone dramatically amplifies this process. The kidneys reabsorb much more sodium than normal (decreasing sodium in urine) while simultaneously secreting much more potassium (increasing potassium in urine). This is exactly what option B describes. Option A incorrectly suggests aldosterone increases sodium excretion—the opposite of its actual effect. Option C wrongly implies aldosterone increases sodium excretion, though it correctly identifies increased potassium excretion. Option D incorrectly suggests decreased potassium excretion, when aldosterone actually promotes potassium loss. Remember the aldosterone trade-off: "sodium in, potassium out." This reciprocal relationship is crucial for understanding not just aldosteronism, but also conditions like Addison's disease (aldosterone deficiency) where you'd see the opposite pattern. Always consider both electrolytes when analyzing mineralocorticoid effects.

Question 8

During the formation of concentrated urine, which combination of processes occurs simultaneously in the collecting duct when ADH levels are high?

  1. Increased water reabsorption and decreased sodium reabsorption from the filtrate
  2. Increased water reabsorption and increased sodium secretion into the filtrate
  3. Increased water reabsorption and increased urea reabsorption from the filtrate (correct answer)
  4. Decreased water reabsorption and increased potassium secretion into the filtrate
  5. Decreased water reabsorption and increased urea secretion into the filtrate
Explanation: When you encounter questions about urine concentration and ADH, focus on how the collecting duct responds to create the most concentrated urine possible while maintaining electrolyte balance. When ADH levels are high, the collecting duct becomes highly permeable to water, dramatically increasing water reabsorption from the filtrate back into the bloodstream. But there's a second crucial process happening simultaneously: increased urea reabsorption. The collecting duct also becomes more permeable to urea, allowing this waste product to be reabsorbed and recycled back into the medullary interstitium. This creates a higher osmotic gradient that helps drive even more water reabsorption, making the urine maximally concentrated. Looking at the wrong answers: Choice A incorrectly suggests decreased sodium reabsorption - but sodium handling in the collecting duct isn't directly affected by ADH and doesn't decrease during urine concentration. Choice B mentions increased sodium secretion, which isn't a primary effect of ADH action. Choice D completely contradicts what happens during ADH action by suggesting decreased water reabsorption, plus potassium secretion isn't the key paired process with water reabsorption during urine concentration. The correct answer is C because it captures both essential simultaneous processes: increased water reabsorption (the primary ADH effect) and increased urea reabsorption (the secondary effect that amplifies concentration). Remember this pattern: ADH doesn't work alone. It coordinates multiple transport processes in the collecting duct to achieve maximum urine concentration through both direct water reabsorption and indirect osmotic effects via urea recycling.

Question 9

A research study measures para-aminohippuric acid (PAH) clearance and inulin clearance simultaneously in a healthy subject. If inulin clearance is 120 mL/min and PAH clearance is 600 mL/min, what can be concluded about the renal handling of PAH?

  1. PAH undergoes passive diffusion because its clearance is proportional to the concentration gradient between plasma and filtrate
  2. PAH undergoes net reabsorption because more PAH is removed from plasma than can be accounted for by filtration alone
  3. PAH clearance data is invalid because no substance can have a clearance greater than the glomerular filtration rate
  4. PAH undergoes net secretion because its clearance exceeds the filtration rate, indicating active transport from peritubular capillaries into tubular lumen (correct answer)
Explanation: When you encounter questions comparing different clearance values, you're being tested on your understanding of how the kidneys handle various substances through filtration, reabsorption, and secretion. The key insight here is comparing PAH clearance (600 mL/min) to inulin clearance (120 mL/min). Inulin is the gold standard for measuring glomerular filtration rate (GFR) because it's freely filtered but neither reabsorbed nor secreted. Since inulin clearance equals GFR, we know 120 mL/min represents the maximum volume that can be filtered. PAH clearance exceeds the filtration rate by 5-fold (600 vs 120 mL/min). This means PAH is being removed from plasma faster than filtration alone could account for. The only way this is possible is if PAH is actively transported from the peritubular capillaries into the tubular lumen—a process called tubular secretion. Option A is incorrect because passive diffusion couldn't explain clearance exceeding GFR; it would result in clearance equal to or less than GFR. Option B confuses the terminology—net reabsorption would decrease clearance below GFR, not increase it above GFR. Option C reflects a fundamental misunderstanding; substances that undergo active secretion routinely have clearances exceeding GFR. Option D correctly identifies that clearance exceeding filtration rate indicates net secretion through active transport. Remember this pattern: clearance less than GFR indicates net reabsorption, clearance equal to GFR indicates no net reabsorption or secretion, and clearance greater than GFR always indicates net secretion.

Question 10

A patient receives an experimental drug that selectively blocks sodium-glucose cotransporters (SGLT) in the proximal tubule while leaving all other transport mechanisms intact. If this patient has normal kidney function and plasma glucose of 100 mg/dL, what would be the most likely consequence?

  1. Compensatory increase in glucose reabsorption through enhanced GLUT transporter activity maintaining normal glucose excretion rates
  2. No significant change in urine composition because glucose reabsorption occurs primarily in the distal convoluted tubule via different transporters
  3. Hypoglycemia due to excessive glucose loss in urine overwhelming hepatic glucose production and dietary glucose absorption
  4. Massive glucosuria with secondary osmotic diuresis leading to dehydration and electrolyte imbalances despite normal plasma glucose levels (correct answer)
Explanation: When you encounter questions about renal transport mechanisms, focus on understanding where specific transporters are located and what happens when they're disrupted. Sodium-glucose cotransporters (SGLT) in the proximal tubule are responsible for reabsorbing virtually all filtered glucose under normal conditions. When you block these transporters completely, glucose that would normally be reabsorbed remains in the tubular fluid and gets excreted in massive amounts - this is called glucosuria. The presence of large amounts of glucose in the urine creates an osmotic effect, pulling water along with it and causing osmotic diuresis. This leads to significant fluid loss, dehydration, and secondary electrolyte imbalances as the body loses both water and essential minerals. Option A is incorrect because GLUT transporters are passive glucose transporters that can't compensate for the active reabsorption normally performed by SGLT proteins. Option B mislocates the primary site of glucose reabsorption - the proximal tubule (not the distal convoluted tubule) handles nearly 100% of glucose reabsorption under normal conditions. Option C overestimates the body's glucose loss; while significant glucosuria occurs, the liver's glucose production and normal dietary intake typically prevent true hypoglycemia in the short term. The correct answer is D because blocking SGLT results in massive glucose loss, osmotic diuresis, dehydration, and electrolyte disturbances. Remember: SGLT inhibitors are actually used clinically as diabetes medications (like empagliflozin) - they work by intentionally causing controlled glucosuria to lower blood glucose levels.

Question 11

An individual with chronic kidney disease has a reduced nephron number but compensatory hyperfiltration in remaining nephrons. If the effective filtration pressure in these remaining glomeruli increases from 10 mmHg to 25 mmHg, while the filtration coefficient remains constant, what would be the expected change in single-nephron GFR?

  1. Single-nephron GFR would increase by approximately 60% due to the nonlinear relationship between pressure and flow across the filtration barrier
  2. Single-nephron GFR would increase by 150% because filtration rate is directly proportional to net filtration pressure (correct answer)
  3. Single-nephron GFR would remain unchanged because autoregulation mechanisms would reduce the filtration coefficient to compensate for increased pressure
  4. Single-nephron GFR would increase by 25% because the change in filtration pressure must be adjusted for the baseline hydrostatic pressure gradient
Explanation: When you encounter questions about glomerular filtration rate (GFR), remember that filtration follows basic principles of fluid dynamics. The key relationship is described by the filtration equation: GFR=Kf×NFPGFR = K_f \times NFP, where KfK_f is the filtration coefficient and NFP is the net filtration pressure. In this scenario, the filtration coefficient remains constant while the effective filtration pressure increases from 10 mmHg to 25 mmHg. Since GFR is directly proportional to filtration pressure when the filtration coefficient is constant, you can calculate the change: 25 mmHg10 mmHg=2.5\frac{25 \text{ mmHg}}{10 \text{ mmHg}} = 2.5, meaning GFR increases by 150% (from 1× to 2.5× the original rate). This confirms answer B is correct. Answer A incorrectly suggests a nonlinear relationship and provides an arbitrary 60% increase that doesn't match the mathematical relationship. The filtration barrier operates linearly within physiological pressure ranges. Answer C misunderstands autoregulation. While autoregulation does occur in healthy kidneys, the question specifically states the filtration coefficient remains constant, and autoregulation primarily affects afferent arteriole resistance, not the filtration coefficient directly. Answer D incorrectly applies a 25% increase, which would only be true if we were looking at the absolute change (15 mmHg increase) relative to some other baseline, not the proportional relationship that governs filtration. Study tip: For GFR calculations, always identify which variables change and which remain constant. When pressure changes and permeability stays the same, expect a directly proportional relationship—double the pressure, double the filtration rate.

Question 12

A patient with heart failure is prescribed a thiazide diuretic that blocks the Na⁺-Cl⁻ cotransporter in the distal convoluted tubule. If this patient maintains normal aldosterone levels, what would be the expected long-term adaptation in the collecting duct?

  1. Increased sodium reabsorption through enhanced epithelial sodium channels (ENaC) activity, partially offsetting the diuretic effect but potentially causing hyperkalemia
  2. Decreased sodium reabsorption with enhanced potassium retention because aldosterone sensitivity is reduced by chronic thiazide exposure
  3. Increased sodium reabsorption through enhanced epithelial sodium channels (ENaC) activity, partially offsetting the diuretic effect while promoting potassium excretion (correct answer)
  4. No significant change in collecting duct function because thiazide diuretics do not affect aldosterone-sensitive transport mechanisms
Explanation: Thiazide diuretics increase sodium delivery to the collecting duct. With normal aldosterone levels, this stimulates ENaC activity, increasing sodium reabsorption and partially offsetting the diuretic effect. However, increased sodium reabsorption through ENaC promotes potassium excretion (not retention), which is why thiazides can cause hypokalemia. Choice A incorrectly suggests hyperkalemia when hypokalemia is expected. Choice B wrongly suggests reduced aldosterone sensitivity. Choice D is incorrect because increased distal sodium delivery definitely affects aldosterone-sensitive mechanisms.

Question 13

A patient with diabetes insipidus has severely reduced ADH secretion. If this patient's glomerular filtration rate remains normal at 120 mL/min, but their ability to concentrate urine is completely impaired, what would be the expected consequence for daily urine volume compared to a healthy individual?

  1. Urine volume would increase to approximately 15-20 L/day because filtrate cannot be concentrated beyond isotonic levels (correct answer)
  2. Urine volume would decrease to approximately 0.5 L/day because compensatory mechanisms increase water reabsorption in the proximal tubule
  3. Urine volume would remain normal at 1-2 L/day because the collecting duct represents only a small fraction of total water reabsorption
  4. Urine volume would increase to approximately 5-8 L/day because aldosterone can partially compensate for the lack of ADH function
Explanation: Without ADH, the collecting duct cannot reabsorb water effectively, leaving urine isotonic with plasma. With normal GFR of 120 mL/min (≈173 L/day of filtrate) and only obligatory reabsorption occurring (no concentration), urine volume increases dramatically to 15-20 L/day. Choice B is wrong because proximal tubule reabsorption is ADH-independent and cannot compensate. Choice C underestimates the collecting duct's role in final concentration. Choice D incorrectly suggests aldosterone affects water reabsorption significantly.

Question 14

An experimental drug selectively inhibits carbonic anhydrase in the proximal tubule. If a patient receives this drug while maintaining normal respiratory function, what would be the expected effects on acid-base balance and electrolyte handling?

  1. Metabolic acidosis with increased sodium and bicarbonate excretion because impaired carbonic anhydrase reduces both H⁺ secretion and HCO₃⁻ reabsorption (correct answer)
  2. Metabolic alkalosis with decreased chloride excretion because carbonic anhydrase inhibition enhances bicarbonate retention in the proximal tubule
  3. No significant acid-base changes because the distal tubule and collecting duct can completely compensate for proximal tubule dysfunction
  4. Respiratory acidosis with hypernatremia because carbonic anhydrase is required for normal CO₂ transport and sodium reabsorption coupling
Explanation: Carbonic anhydrase in proximal tubule cells is essential for converting CO₂ and H₂O to H⁺ and HCO₃⁻. Inhibition reduces H⁺ secretion (decreasing acid excretion) and impairs HCO₃⁻ reabsorption, leading to metabolic acidosis and bicarbonate wasting. Sodium excretion increases because Na⁺-H⁺ exchange is reduced. Choice B incorrectly predicts alkalosis and enhanced bicarbonate retention. Choice C overestimates distal compensation capacity. Choice D wrongly suggests respiratory acidosis when the problem is metabolic.

Question 15

A patient with severe dehydration has maximally concentrated urine (1200 mOsm/kg) and elevated plasma osmolality (310 mOsm/kg). If this patient's ADH levels are maximally elevated, what is the primary mechanism allowing the kidney to produce urine more concentrated than plasma?

  1. Active water pumps in the collecting duct directly transport water against its concentration gradient when stimulated by ADH
  2. Countercurrent multiplication by the loop of Henle creates a hypertonic medullary interstitium, allowing water reabsorption without solute in the ADH-sensitive collecting duct (correct answer)
  3. Selective secretion of excess solutes into the tubular lumen concentrates the remaining water without requiring water reabsorption
  4. Increased glomerular filtration of water relative to solutes provides a dilute filtrate that becomes concentrated through normal reabsorption processes
Explanation: When you encounter questions about urine concentration, focus on how the kidney can produce urine that's more concentrated than blood plasma - a seemingly impossible task that requires understanding the countercurrent multiplication system. The kidney achieves maximum urine concentration through a two-step process. First, the loop of Henle creates a gradient of increasing osmolality from the cortex (300 mOsm/kg) down to the inner medulla (up to 1200 mOsm/kg). The descending limb is permeable to water but not solutes, while the ascending limb actively pumps out sodium and chloride without water following. This countercurrent flow multiplies the concentration gradient. Second, when ADH is present, it makes the collecting duct permeable to water, allowing water to be reabsorbed into the hypertonic medullary interstitium without solute reabsorption, concentrating the urine. Answer B correctly describes this mechanism - the hypertonic medullary interstitium created by countercurrent multiplication provides the driving force for water reabsorption in the ADH-sensitive collecting duct. Answer A is wrong because there are no active water pumps; water moves passively down osmotic gradients. Answer C incorrectly suggests solute secretion is the primary mechanism, when water reabsorption is key. Answer D misrepresents the process - the filtrate starts isotonic to plasma, and concentration occurs through selective water reabsorption, not differential filtration. Remember: urine concentration depends on the medullary osmotic gradient plus ADH-mediated water permeability in the collecting duct. Without either component, maximum concentration is impossible.

Question 16

A research study measures the handling of substance X by the kidneys. The data shows that substance X has a clearance rate that exceeds the glomerular filtration rate. Based on the diagram showing renal clearance patterns, what can be concluded about the renal handling of substance X?

  1. Substance X is freely filtered and undergoes net tubular reabsorption throughout the nephron
  2. Substance X is freely filtered and undergoes net tubular secretion by the nephron tubules (correct answer)
  3. Substance X is neither filtered nor processed by the tubules, indicating no renal handling
  4. Substance X is filtered but completely reabsorbed before reaching the collecting duct
  5. Substance X undergoes glomerular secretion in addition to normal filtration processes
Explanation: When clearance exceeds GFR, it indicates net tubular secretion. The kidney removes more of the substance from plasma than can be accounted for by filtration alone, meaning tubular cells actively transport the substance from blood into urine. A is incorrect because reabsorption would decrease clearance below GFR. C is wrong because high clearance indicates active processing. D is incorrect because complete reabsorption would result in zero clearance. E is wrong because secretion occurs in tubules, not glomeruli.