Acid–Base And Electrolyte Regulation - USMLE Step 1

1. Which of the following is the most likely mechanism underlying this patient's hyponatremia?
   Answer: Ectopic ADH secretion leading to excessive free water retention This patient presents with euvolemic hyponatremia, characterized by low serum sodium and osmolality in the setting of inappropriately concentrated urine (urine osmolality > 100 mOsm/kg). This clinical picture, especially in a patient with small cell lung cancer, is classic for the Syndrome of Inappropriate Antidiuretic Hormone (SIADH). Small cell lung cancer can ectopically produce ADH, which acts on the collecting ducts to increase aquaporin channels, leading to excessive free water reabsorption and dilutional hyponatremia.
2. The refractory hypocalcemia in this patient is best explained by the low magnesium level's effect on which of the following?
   Answer: Impairing parathyroid hormone secretion and action Severe hypomagnesemia is a common cause of refractory hypocalcemia, particularly in patients with alcoholism. Magnesium is required for the normal functioning of the calcium-sensing receptor and for the secretion of parathyroid hormone (PTH). Severe hypomagnesemia impairs PTH release from the parathyroid glands. Furthermore, it causes end-organ resistance to the effects of PTH at the bone and kidney. This combination of decreased PTH secretion and peripheral PTH resistance leads to hypocalcemia that will not correct until the magnesium deficiency is addressed.
3. Which of the following nephron segments is responsible for reabsorbing approximately 85% of the filtered load of bicarbonate?
   Answer: Proximal convoluted tubule The proximal convoluted tubule is the primary site for bicarbonate (HCO3-) reabsorption, reclaiming about 85% of the filtered load. This process is dependent on the enzyme carbonic anhydrase and the Na+/H+ exchanger on the apical membrane. H+ is secreted into the lumen, where it combines with filtered HCO3- to form H2CO3. Carbonic anhydrase on the brush border converts H2CO3 to CO2 and H2O, which diffuse into the cell. Inside the cell, cytoplasmic carbonic anhydrase reverses the reaction, and the regenerated HCO3- is transported across the basolateral membrane into the blood.
4. In addition to its vasoconstrictive effects, angiotensin II directly promotes sodium and water retention by stimulating which of the following transport processes in the proximal convoluted tubule?
   Answer: Na+/H+ exchange Angiotensin II has multiple effects on the kidney to increase sodium and water retention. One of its key direct effects is on the proximal convoluted tubule, where it stimulates the activity of the Na+/H+ exchanger (NHE3) on the apical membrane. This increases sodium reabsorption from the tubular fluid into the cell. The associated H+ secretion also facilitates bicarbonate reabsorption via the action of carbonic anhydrase. This leads to increased reabsorption of sodium, bicarbonate, and osmotically obligated water, contributing to volume expansion.
5. Spironolactone achieves its potassium-sparing effect by acting as an antagonist at which of the following receptors?
   Answer: Mineralocorticoid receptor Spironolactone is a potassium-sparing diuretic that functions as a competitive antagonist of the mineralocorticoid (aldosterone) receptor in the collecting duct. In patients with cirrhosis, secondary hyperaldosteronism contributes to sodium and water retention. By blocking the action of aldosterone, spironolactone prevents the transcription and insertion of epithelial sodium channels (ENaC) and Na+/K+-ATPases in principal cells. This reduces sodium reabsorption and, consequently, decreases the luminal negative potential that drives potassium secretion, thus 'sparing' potassium.
6. The elevated parathyroid hormone level in this patient is primarily a response to which of the following renal abnormalities?
   Answer: Decreased production of 1,25-dihydroxyvitamin D This patient has secondary hyperparathyroidism due to chronic kidney disease (CKD). In CKD, two major factors stimulate PTH secretion. First, the failing kidneys are unable to excrete phosphate, leading to hyperphosphatemia. Second, and more importantly, the damaged renal parenchyma has reduced activity of 1-alpha-hydroxylase, the enzyme that converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol). Low calcitriol levels lead to decreased intestinal calcium absorption, causing hypocalcemia. Both hypocalcemia and the lack of negative feedback from calcitriol on the parathyroid glands lead to a massive, sustained increase in PTH secretion.
7. This patient's condition is most likely caused by a defect in which of the following renal processes?
   Answer: Distal tubular hydrogen ion secretion This patient presents with a classic triad for distal (Type 1) Renal Tubular Acidosis (RTA): non-anion gap metabolic acidosis, hypokalemia, and an inability to acidify the urine (urine pH > 5.5). The underlying defect is in the α-intercalated cells of the distal tubule and collecting duct, which are unable to secrete H+ into the lumen. This impairs acid excretion and bicarbonate generation, leading to systemic acidosis. Chronic acidosis leads to calcium release from bone and hypercalciuria, which, combined with the alkaline urine, promotes the formation of calcium phosphate stones.
8. The patient's neurologic symptoms are most directly related to a decrease in the serum concentration of which of the following?
   Answer: Ionized calcium The patient is hyperventilating due to anxiety, leading to respiratory alkalosis (low PaCO2, high pH). In a state of alkalemia, albumin and other plasma proteins become more negatively charged, which increases their binding of positively charged ions, particularly calcium. This leads to a decrease in the physiologically active, ionized form of calcium (Ca2+), even though the total serum calcium concentration remains unchanged. Hypocalcemia increases neuronal excitability, causing the characteristic perioral numbness, paresthesias, and potentially tetany (carpopedal spasm).
9. Which of the following best describes the acid-base disturbances present in this patient?
   Answer: Metabolic acidosis and respiratory alkalosis Salicylate (aspirin) overdose classically causes a mixed acid-base disorder. Initially, salicylates directly stimulate the medullary respiratory center, causing hyperventilation and a primary respiratory alkalosis (low PaCO2). Subsequently, salicylates uncouple oxidative phosphorylation and inhibit enzymes in the Krebs cycle, leading to the accumulation of organic acids (ketoacids, lactate) and a primary anion gap metabolic acidosis (low HCO3-). The patient's labs show a low PaCO2 (respiratory alkalosis) and a low HCO3- (metabolic acidosis). The near-normal pH reflects the opposing effects of the two primary disorders.
10. In addition to the intended effect on blood pressure, which of the following sets of electrolyte abnormalities is most likely to be seen in this patient?
    Answer: Hypokalemia and hypercalcemia Hydrochlorothiazide is a thiazide diuretic that inhibits the Na+/Cl- cotransporter in the distal convoluted tubule. This leads to several characteristic electrolyte disturbances. The increased delivery of sodium to the collecting duct promotes potassium secretion, leading to hypokalemia. Uniquely among diuretics, thiazides increase calcium reabsorption in the distal tubule by enhancing the activity of the basolateral Na+/Ca2+ exchanger, which can lead to hypercalcemia. Other common side effects include hyponatremia and hyperuricemia.
11. The medication recently started is most likely causing this patient's hyperkalemia by which of the following mechanisms?
    Answer: Reduced angiotensin II production leading to decreased aldosterone secretion Lisinopril is an ACE inhibitor, which blocks the conversion of angiotensin I to angiotensin II. Reduced angiotensin II levels lead to decreased secretion of aldosterone from the adrenal cortex. Aldosterone normally promotes potassium secretion in the distal nephron by upregulating the epithelial sodium channel (ENaC) and the renal outer medullary potassium channel (ROMK). Therefore, ACE inhibitor therapy can lead to hyperkalemia by reducing aldosterone-mediated potassium excretion.
12. The metabolic acidosis in this patient is primarily due to a defect in the reabsorption of which substance in the proximal tubule?
    Answer: Bicarbonate Fanconi syndrome is a generalized dysfunction of the proximal convoluted tubule. This leads to impaired reabsorption of numerous substances, including bicarbonate, glucose, amino acids, and phosphate. The inability to reabsorb the filtered load of bicarbonate results in significant bicarbonate wasting in the urine. This loss of base from the body is the primary cause of the normal anion gap metabolic acidosis seen in proximal (Type 2) RTA, which is a key feature of Fanconi syndrome.
13. Which of the following best explains the finding of an acidic urine in this patient with metabolic alkalosis?
    Answer: Volume contraction stimulating aldosterone and H+ secretion This patient has metabolic alkalosis from vomiting (loss of HCl). The associated volume depletion activates the renin-angiotensin-aldosterone system. Aldosterone increases sodium reabsorption in the collecting duct at the expense of potassium and H+ secretion. The hypokalemia also contributes by promoting H+ secretion in exchange for K+ reabsorption via the H+/K+-ATPase in intercalated cells. This results in excretion of an acidic urine despite systemic alkalosis, a phenomenon known as 'paradoxical aciduria.'
14. This patient's hypernatremia is a direct consequence of impaired reabsorption of which of the following substances in the collecting ducts?
    Answer: Water The clinical presentation of polyuria, hypernatremia, high serum osmolality, and inappropriately dilute urine is characteristic of diabetes insipidus (DI). In the context of head trauma, this is most likely central DI, caused by damage to the hypothalamus or posterior pituitary, leading to deficient ADH secretion. ADH normally promotes free water reabsorption in the collecting ducts by increasing the insertion of aquaporin-2 channels. Without ADH, the collecting ducts are impermeable to water, leading to the excretion of a large volume of dilute urine and a subsequent increase in serum sodium concentration due to pure water loss.
15. This combination of findings is most consistent with a disorder involving which of the following?
    Answer: Decreased synthesis or effect of aldosterone This patient has Type 4 Renal Tubular Acidosis, which is characterized by hyperkalemia and a normal anion gap metabolic acidosis. This is the most common form of RTA and is often seen in patients with diabetes and chronic kidney disease. The underlying pathophysiology is either deficient aldosterone production (hyporeninemic hypoaldosteronism) or resistance to its effects in the collecting duct. Aldosterone is necessary for both potassium and hydrogen ion secretion. Its deficiency or resistance leads to retention of both K+ and H+, causing hyperkalemia and acidosis.
16. Which of the following sets of physiological changes is most likely to be found in this individual upon rescue?
    Answer: Increased plasma osmolality, increased ADH, increased urine osmolality Dehydration leads to a loss of free water, which concentrates the solutes in the blood, causing an increase in plasma osmolality. This change is sensed by osmoreceptors in the hypothalamus, which stimulates both the sensation of thirst and the release of antidiuretic hormone (ADH) from the posterior pituitary. ADH travels to the kidneys and acts on the collecting ducts, increasing their permeability to water. This allows for maximal water reabsorption from the filtrate, resulting in the production of a small volume of highly concentrated urine (high urine osmolality) to conserve body water.
17. Furosemide causes potassium wasting primarily by inhibiting which of the following transporters in the renal tubules?
    Answer: Na+/K+/2Cl- cotransporter in the thick ascending limb of the loop of Henle Furosemide is a loop diuretic that acts by inhibiting the Na+/K+/2Cl- (NKCC2) cotransporter in the apical membrane of cells in the thick ascending limb of the loop of Henle. This inhibition reduces the reabsorption of these ions, leading to significant natriuresis and diuresis. The blockade also disrupts the generation of the positive luminal potential, which normally drives paracellular reabsorption of cations like Ca2+ and Mg2+. The increased delivery of sodium to the distal nephron stimulates the renin-angiotensin-aldosterone system and enhances potassium secretion in the collecting ducts, leading to hypokalemia.
18. In response to this patient's acid-base disturbance, which of the following represents the primary renal compensatory mechanism?
    Answer: Increased secretion of H+ and generation of new bicarbonate The patient is in diabetic ketoacidosis, which causes a high anion gap metabolic acidosis. The primary renal compensation for metabolic acidosis is to increase the excretion of acid and conserve bicarbonate. This is achieved by increasing the secretion of H+ ions into the tubular fluid by the α-intercalated cells of the collecting duct and the proximal tubule cells. For each H+ secreted, a new bicarbonate ion is generated and returned to the blood, helping to correct the acidosis.
19. If his PaCO2 remains elevated, which of the following changes in renal function is expected to occur over the next several days to compensate for this acid-base disorder?
    Answer: Increased renal bicarbonate reabsorption and generation The patient has acute respiratory acidosis due to CO2 retention from his COPD exacerbation. The appropriate physiological compensation is renal. Over several days, the kidneys will increase their net acid excretion. This involves two main processes: increased reclamation of all filtered bicarbonate and increased generation of new bicarbonate, primarily through enhanced ammonium (NH4+) synthesis and secretion by proximal tubule cells and increased H+ secretion. This leads to a rise in serum bicarbonate, which helps to normalize the pH.
20. The electrolyte and acid-base abnormalities in this patient are caused by the direct action of aldosterone on which part of the nephron?
    Answer: Principal and intercalated cells of the collecting duct This patient has primary hyperaldosteronism (Conn's syndrome). Aldosterone's primary site of action is the late distal tubule and the collecting duct. It acts on the principal cells to increase the expression of apical epithelial sodium channels (ENaC) and basolateral Na+/K+-ATPases, promoting sodium reabsorption and potassium secretion. It also acts on the α-intercalated cells to stimulate H+ secretion via the H+-ATPase. The combined effect of increased K+ and H+ secretion leads to the characteristic findings of hypokalemia and metabolic alkalosis.

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