A 25-year-old woman presents to the emergency department with symptoms of nausea and vomiting of 2 days duration. She is not on any medications and was previously well until now. The physical examination is normal except for a postural drop in her blood pressure from 110/80 mm Hg supine to 90/80 mm Hg standing. Her serum electrolytes are sodium 130 mEq/L, potassium 3 mEq/L, chloride 90 mEq/L, bicarbonate 30 mEq/L, urea 50 mg/dL, and creatinine 0.8 mg/dL. Which of the following electrolytes is most likely to be filtered through the glomerulus but unaffected by tubular secretion?
High-Yield Explanation
Urea is filtered at the glomerulus, and thereafter, any movement in or out of tubules is a passive process depending on gradients, not secretion. Reabsorption of urea in the distal tubule and collecting duct, when urine flow is reduced, results in the disproportionate elevation of urea nitrogen over creatinine in prerenal azotemia.