Renal System MCAT Practice Question
A 68-year-old man with stage 3b chronic kidney disease (eGFR 28 mL/min/1.73m²) presents for routine follow-up. Laboratory studies show serum potassium 6.1 mEq/L (normal 3.5-5.0), serum sodium 138 mEq/L, serum creatinine 2.8 mg/dL, and plasma renin activity and aldosterone levels that are appropriately elevated in response to the hyperkalemia. Urine potassium excretion is inappropriately low at 15 mEq/day (normal 40-120 mEq/day). An ECG shows peaked T waves. The patient denies use of ACE inhibitors, ARBs, NSAIDs, or potassium supplements. Which of the following best explains the mechanism of potassium retention in this patient despite appropriate hormonal responses?
Answer choices
- ADecreased filtered load of potassium at the glomerulus due to reduced GFR
- BImpaired Na+ reabsorption in the collecting duct principal cells reducing the negative electrical gradient favoring K+ secretionCorrect answer
- CChronic metabolic acidosis causing increased cellular potassium uptake via Na-K-ATPase
- DAldosterone resistance of principal cells resulting in inability to upregulate ENaC expression
- ELoss of renal mass reducing the absolute number of functioning collecting ducts available for potassium secretion
- FEnhanced potassium reabsorption in the proximal tubule and thick ascending limb limiting distal delivery
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