A substance is freely filtered at the glomerulus. Its plasma concentration is 1 mg/mL, its urine concentration is 60 mg/mL, and the urine flow rate is 1 mL/min. If GFR is 120 mL/min, this substance is best characterized as:

• A Freely filtered only, with no tubular secretion or reabsorption
• B Filtered and secreted
• C Protein-bound and not filtered
• D Filtered and partially reabsorbed ✓

Explanation

Filtered load = GFR × Plasma concentration = 120 × 1 = 120 mg/min. Excreted amount = Urine flow × Urine concentration = 1 × 60 = 60 mg/min. Since excreted < filtered, net tubular reabsorption has occurred. If excreted = filtered, no net transport; if excreted > filtered, net secretion is occurring. This pattern is similar to that of glucose under high-load conditions.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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