Acid–Base And Electrolyte Disorders - USMLE Step 2

1. Which of the following ECG findings is most characteristic of the electrolyte abnormality expected in this patient?
   Answer: Broad, flattened T waves and U waves Severe diarrhea is a common cause of hypokalemia due to large gastrointestinal losses of potassium. The characteristic ECG findings of hypokalemia include flattening or inversion of the T wave, the appearance of a U wave (a positive deflection following the T wave), ST-segment depression, and QT interval prolongation. Peaked T waves are classic for hyperkalemia. A shortened QT interval is seen with hypercalcemia. ST-segment elevation is seen in acute myocardial infarction or pericarditis.
2. The addition of which medication most likely precipitated her hyperkalemia?
   Answer: Spironolactone This patient has multiple risk factors for hyperkalemia, including chronic kidney disease and the use of an ACE inhibitor (lisinopril). The most likely precipitant for her acute worsening is the recent addition of spironolactone. Spironolactone is a potassium-sparing diuretic that acts as an aldosterone antagonist. Aldosterone normally promotes potassium excretion; by blocking its effect, spironolactone can cause significant potassium retention. The combination of an ACE inhibitor (which also increases potassium by reducing aldosterone production) and spironolactone in a patient with underlying CKD is a well-known cause of severe hyperkalemia. Furosemide is a loop diuretic that causes potassium wasting.
3. What is the generally accepted maximum rate of potassium chloride infusion through a peripheral intravenous line to minimize the risk of phlebitis and pain?
   Answer: 10 mEq/hour Intravenous potassium chloride is caustic to peripheral veins and can cause significant pain, phlebitis, and tissue necrosis if it extravasates. To minimize these risks, the rate of infusion through a peripheral line should generally not exceed 10 mEq/hour. The concentration should also be limited, typically to no more than 40 mEq/L. Higher rates (e.g., 20 mEq/hour or more) require a central venous catheter and continuous cardiac monitoring due to the risk of inducing life-threatening hyperkalemia and arrhythmias.
4. Which of the following is the most likely diagnosis?
   Answer: Type 1 (distal) renal tubular acidosis This patient has a normal anion gap metabolic acidosis (NAGMA) calculated as 140 - (112 + 16) = 12. The presence of hypokalemia and NAGMA suggests either diarrhea or a renal tubular acidosis (RTA). The inappropriately high urine pH (>5.5) in the setting of systemic acidosis is the hallmark of Type 1 (distal) RTA, which is caused by impaired H⁺ secretion in the distal tubule. Sjögren syndrome is a known cause of Type 1 RTA. Type 2 RTA involves impaired bicarbonate reabsorption and the urine can be acidified. Type 4 RTA is associated with hyperkalemia. Diarrhea would cause a NAGMA with hypokalemia, but the urine would be appropriately acidic (pH <5.5).
5. Which of the following is the most appropriate next step in management?
   Answer: Provide reassurance and have her rebreathe into a paper bag The patient's symptoms (perioral and digital paresthesias, lightheadedness) and ABG results (high pH, low pCO₂) are classic for acute respiratory alkalosis caused by hyperventilation, likely secondary to a panic attack given her history of stress. The paresthesias are caused by decreased ionized calcium due to the alkalosis. The most appropriate initial management is to calm the patient and address the hyperventilation. Having the patient rebreathe into a paper bag increases the inspired CO₂, which helps correct the respiratory alkalosis and alleviates the symptoms. While a pulmonary embolism can cause respiratory alkalosis, her presentation is much more consistent with a panic attack, making CT angiography unnecessary at this point. IV lorazepam is too aggressive for a first step. A non-rebreather mask would worsen the alkalosis.
6. In addition to diuretic therapy, which of the following is the most important intervention for managing his hyponatremia?
   Answer: Water and salt restriction This patient has hypervolemic hyponatremia secondary to severe congestive heart failure. The low cardiac output leads to activation of the renin-angiotensin-aldosterone system and release of ADH, causing retention of both salt and, disproportionately, water. This results in a dilutional hyponatremia despite an increase in total body sodium. The primary management involves treating the underlying heart failure (e.g., with diuretics) and restricting both water and sodium intake. Giving saline or sodium tablets would worsen the volume overload. Demeclocycline is a second-line therapy for SIADH and is not typically used in heart failure.
7. Which of the following medications is most likely responsible for these findings?
   Answer: Hydrochlorothiazide Thiazide diuretics, such as hydrochlorothiazide, are a common cause of both hypokalemia and hypomagnesemia. They inhibit the Na-Cl cotransporter in the distal convoluted tubule, leading to increased distal delivery of sodium and water. This enhances potassium secretion in the collecting duct, causing hypokalemia. Thiazides also increase the urinary excretion of magnesium, leading to hypomagnesemia. ACE inhibitors (lisinopril) and ARBs (losartan) typically cause hyperkalemia. Calcium channel blockers like amlodipine do not have a significant effect on potassium or magnesium levels.
8. Which of the following is the most appropriate initial treatment for this patient's hyponatremia?
   Answer: Fluid restriction This patient's presentation is classic for the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which is a common paraneoplastic syndrome associated with small cell lung cancer. The diagnosis is supported by euvolemic hyponatremia, low serum osmolality, inappropriately concentrated urine (urine osmolality > 100 mOsm/kg), and elevated urine sodium (>40 mEq/L). The patient is asymptomatic or has mild symptoms (fatigue, nausea). The first-line treatment for mild to moderate SIADH is fluid restriction. This reduces the free water intake, allowing the serum sodium to rise gradually. 0.9% saline can worsen hyponatremia in SIADH. 3% saline is reserved for severe symptoms like seizures. Desmopressin is used to treat central diabetes insipidus.
9. Which of the following best describes this patient's acid-base disturbance?
   Answer: Metabolic acidosis and respiratory alkalosis This patient has sepsis-induced lactic acidosis causing a high anion gap metabolic acidosis (low pH, low HCO₃⁻, elevated lactate). Sepsis also directly stimulates the respiratory center, causing a primary respiratory alkalosis (low pCO₂) independent of metabolic compensation. This mixed disorder - metabolic acidosis with concurrent respiratory alkalosis - is classic in sepsis. Using Winter's formula for pure metabolic acidosis compensation: Expected pCO₂ = (1.5 × 9) + 8 = 21.5 mm Hg. The actual pCO₂ of 24 mm Hg is close but the clinical context of sepsis makes a mixed disorder most likely, as sepsis independently causes respiratory alkalosis through direct CNS stimulation.
10. Overly rapid correction of her serum sodium concentration places her at the highest risk for which of the following complications?
    Answer: Cerebral edema In chronic hypernatremia, brain cells accumulate intracellular solutes (idiogenic osmoles) to prevent cellular dehydration. If the serum sodium is corrected too rapidly with hypotonic fluids, the extracellular fluid becomes hypotonic relative to the brain cells. This creates an osmotic gradient that drives water into the brain cells, leading to cerebral edema. This can cause seizures, permanent neurologic injury, and death. Osmotic demyelination syndrome is associated with overly rapid correction of hyponatremia, not hypernatremia.
11. Which of the following interventions is most likely to confirm the suspected diagnosis?
    Answer: Administration of desmopressin This patient's presentation of hypernatremia, polyuria, and inappropriately dilute urine following a head injury is highly suggestive of central diabetes insipidus (DI), caused by decreased ADH secretion. A water deprivation test is the classic diagnostic test but may be dangerous in a critically ill patient with high urine output. A more practical approach in this setting is to administer a therapeutic and diagnostic trial of desmopressin (an ADH analog). In central DI, administration of desmopressin will lead to a rapid decrease in urine output and an increase in urine osmolality, confirming the diagnosis. In nephrogenic DI, there would be no significant response.
12. Which of the following is the most appropriate initial fluid management?
    Answer: Intravenous 0.9% saline This patient has hypovolemic hyponatremia. Her clinical signs (lethargy, dry mucous membranes, hypotension, tachycardia) point to volume depletion. The hyponatremia is likely caused by her hydrochlorothiazide use, which impairs the kidney's ability to dilute urine and causes volume loss. The low urine sodium (<20 mEq/L) is consistent with the kidneys trying to conserve sodium in a volume-depleted state. The initial treatment for hypovolemic hyponatremia is to restore volume with isotonic (0.9%) saline. This will correct the hypovolemia, which will turn off the non-osmotic ADH stimulus, allowing the kidneys to excrete free water and correct the sodium. 3% saline is for severe, symptomatic hyponatremia. Water restriction is for euvolemic (SIADH) or hypervolemic hyponatremia. D5W would worsen the hyponatremia.
13. To minimize the risk of osmotic demyelination syndrome, the serum sodium level should not be corrected by more than how much in the first 24 hours?
    Answer: 8 mEq/L Rapid correction of chronic hyponatremia (>48 hours duration) can lead to osmotic demyelination syndrome (formerly central pontine myelinolysis), a devastating neurologic condition. Brain cells adapt to chronic hyponatremia by extruding osmolytes to prevent cerebral edema. If the serum sodium is raised too quickly, water will shift out of the brain cells, leading to their shrinkage and demyelination. Current guidelines recommend a correction rate of no more than 8-10 mEq/L in any 24-hour period, and no more than 18 mEq/L in any 48-hour period. In high-risk patients (like this one with alcoholism), a slower rate of 4-6 mEq/L per 24 hours is often targeted.
14. Which of the following is the most appropriate initial intravenous fluid for resuscitation?
    Answer: 0.9% saline This patient has severe hypernatremia and is hemodynamically unstable (hypotensive, tachycardic), indicating significant hypovolemia. The immediate priority in any patient with circulatory collapse is to restore intravascular volume. Isotonic (0.9%) saline is the fluid of choice for initial resuscitation in hypovolemic patients, regardless of their serum sodium concentration. Once the patient is hemodynamically stable, the focus can shift to correcting the free water deficit using hypotonic fluids like 0.45% saline or D5W. Starting with hypotonic fluids in a hypotensive patient could worsen the hypotension. 3% saline is used for hyponatremia and is contraindicated here.
15. The refractory nature of his hypokalemia is most likely due to a deficiency of which of the following?
    Answer: Magnesium Hypomagnesemia is a common finding in patients with alcohol use disorder and is a frequent cause of refractory hypokalemia. Magnesium is a crucial cofactor for the Na-K-ATPase pump, which maintains the intracellular potassium concentration. Additionally, magnesium blocks the renal outer medullary potassium (ROMK) channels in the collecting duct, preventing potassium secretion. In a state of hypomagnesemia, this inhibition is lost, leading to continuous renal potassium wasting. Therefore, in a patient with refractory hypokalemia, especially with risk factors like alcoholism, magnesium levels must be checked and repleted before potassium can be effectively corrected.
16. What is the next most appropriate step to rapidly lower his serum potassium level?
    Answer: Administer intravenous regular insulin with dextrose After stabilizing the cardiac membrane with calcium, the next priority is to rapidly shift potassium from the extracellular to the intracellular space. The most effective and rapid way to achieve this is by administering intravenous regular insulin followed by dextrose (to prevent hypoglycemia). Insulin activates the Na-K-ATPase pump, driving potassium into cells. This intervention begins to work within 10-20 minutes. Beta-2 agonists like albuterol also shift potassium intracellularly. Loop diuretics and cation exchange resins (like patiromer or sodium polystyrene sulfonate) remove potassium from the body but work more slowly. Hemodialysis is the most effective way to remove potassium but is reserved for severe cases or when other measures fail, and it takes time to arrange.
17. What is her corrected serum sodium concentration?
    Answer: 139 mEq/L Severe hyperglycemia creates an osmotic gradient that pulls water from the intracellular space into the extracellular space, diluting the serum sodium. This is a form of hypertonic hyponatremia. To estimate the true sodium level, a correction factor is used. For every 100 mg/dL increase in glucose above 100 mg/dL, the serum sodium decreases by approximately 1.6 mEq/L. The glucose elevation is 650 mg/dL above normal (750-100). The correction is (650/100) × 1.6 = 10.4 mEq/L. Corrected sodium = 128 + 10.4 = 138.4 mEq/L, which is closest to 139 mEq/L. This corrected value reflects the sodium concentration that will be present once the hyperglycemia is resolved with insulin therapy.
18. Which of the following is the most appropriate immediate step in management?
    Answer: Administer intravenous calcium gluconate This patient has severe hyperkalemia with significant ECG changes (peaked T waves, PR prolongation, QRS widening), which indicate cardiac membrane instability and a high risk of life-threatening arrhythmias. The most important first step is to stabilize the cardiac membrane. Intravenous calcium gluconate (or calcium chloride) is administered for this purpose. It does not lower the serum potassium level but directly antagonizes the toxic effects of hyperkalemia on the myocyte cell membrane, preventing arrhythmias. After membrane stabilization, therapies to shift potassium into cells (insulin/glucose, albuterol, sodium bicarbonate) and to remove potassium from the body (diuretics, cation exchange resins, hemodialysis) can be initiated.
19. According to Winter's formula, what is the expected partial pressure of carbon dioxide (pCO₂) as a compensatory response?
    Answer: 26 mm Hg In a primary metabolic acidosis, the respiratory system compensates by hyperventilating to 'blow off' CO₂, thereby raising the pH. Winter's formula is used to predict the expected degree of respiratory compensation. The formula is: Expected pCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. In this case, Expected pCO₂ = (1.5 × 12) + 8 = 18 + 8 = 26 mm Hg. The expected range would be 24-28 mm Hg. Therefore, 26 mm Hg is the expected pCO₂.
20. This patient's acid-base status is most consistent with toxicity from which substance?
    Answer: Salicylates Salicylate (aspirin) toxicity classically causes a mixed acid-base disorder. Salicylates directly stimulate the medullary respiratory center, leading to hyperventilation and a primary respiratory alkalosis (low pCO₂). They also uncouple oxidative phosphorylation, leading to the accumulation of organic acids (ketoacids, lactate) and a primary high anion gap metabolic acidosis (low HCO₃⁻). This patient's ABG shows a near-normal pH, a significantly low pCO₂ (respiratory alkalosis), and a significantly low HCO₃⁻ (metabolic acidosis), which is the hallmark of this mixed disorder. Ethylene glycol and methanol cause a profound metabolic acidosis without a primary respiratory alkalosis. Benzodiazepines would cause respiratory acidosis from respiratory depression.

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