<prompt> 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 11080 mm Hg supine to 9080 mm Hg standing. Her serum electrolytes are sodium 130 mEqL, potassium 3 mEqL, chloride 90 mEqL, bicarbonate 30 mEqL, urea 50 mgdL, and creatinine 0.8 mgdL. Which of the following electrolytes is most likely to be filtered through the glomerulus but unaffected by tubular secretion?potassiumsodiumbicarbonateurea  <response>: 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. 