Renal System MCAT Practice Question
A 68-year-old man with a history of diabetes mellitus presents for evaluation of polyuria and polydipsia. During clinical assessment, inulin is infused intravenously and urine is collected over 2 hours. Inulin is freely filtered at the glomerulus, not reabsorbed or secreted by the tubules, and is physiologically inert. Plasma inulin concentration is measured at 2 mg/dL, and urine inulin concentration is 150 mg/dL with a urine flow rate of 10 mL/min. Which of the following parameters can be most directly calculated from these measurements to assess this patient's renal function?
Answer choices
- AEffective renal plasma flow, which would help determine if the patient has renovascular disease
- BGlomerular filtration rate, which reflects the number of functioning nephrons and kidney's excretory capacityCorrect answer
- CRenal free water clearance, which indicates the kidney's ability to dilute or concentrate urine independent of solute excretion
- DTubular maximum for glucose reabsorption, which would explain the polyuria in this diabetic patient
- EPeritubular capillary hydrostatic pressure, which determines the driving force for tubular reabsorption
- FFractional excretion of sodium, which would indicate whether the polyuria is due to osmotic diuresis
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