Acid-Base Disorders USMLE Step 1 Practice Question
A 38-year-old woman presents to the clinic with a 3-month history of progressive headaches, polyuria, and muscle weakness. Her blood pressure is 162/98 mmHg. Laboratory studies show: Serum electrolytes:
- Na+ 148 mEq/L
- K+ 2.8 mEq/L
- Cl- 102 mEq/L
- HCO3- 38 mEq/L Arterial blood gas:
- pH 7.48
- PaCO2 48 mmHg
- PaO2 95 mmHg Plasma aldosterone 24 ng/dL (normal <12 ng/dL)
Plasma renin activity 0.3 ng/mL/hr (normal 0.6-3.0 ng/mL/hr)
Urine chloride 28 mEq/L
Urine output 2.5 L/day Which of the following best explains why the urine chloride level is elevated in this patient?
Answer choices
- AImpaired proximal tubule reabsorption of sodium and chloride due to metabolic acidosis
- BResistance of the collecting duct to aldosterone, preventing sodium reabsorption and chloride retention
- CContinued delivery of sodium and chloride to the distal tubule due to aldosterone-mediated volume expansion, exceeding the tubule's reabsorptive capacityCorrect answer
- DLoss of gastric hydrochloric acid from recurrent vomiting or nasogastric suction
- EAcute kidney injury with inability to concentrate urine appropriately
- FHyperkalemia-induced increase in distal tubular secretion of chloride
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