Question
The clearance of substances X, Y, Z are studied at different concentrations in the blood (diagram below). Which of the following is correctly matched?
| A. |
X- glucose, Y- HCO3-, Z- Inulin
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| B. |
X- Inulin, Y- HCO3-, Z- Inulin
|
| C. |
X- glucose, Y- HCO3-, Z- PAH
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| D. |
X- Inulin, Y- PAH, Z- glucose
|
Show Answer
|
Correct Answer � D
Explanation
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– X (flat line ~125 mL/min): Indicates Inulin clearance, which is constant and independent of plasma concentration. Inulin is freely filtered, not secreted or reabsorbed. Used to measure glomerular filtration rate (GFR).
– Y (high clearance initially, then decreases as plasma level rises): Indicates PAH (para-aminohippurate). It is filtered and actively secreted by tubules. Clearance exceeds GFR at low concentrations and plateaus when secretion transporters saturate.
– Z (clearance starts at 0, increases slightly but remains low): Matches Glucose, which is completely reabsorbed at low concentrations. Clearance is zero until renal threshold (~180 mg/dL) is exceeded; then glucose appears in the urine.
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