## **Core Concept**
The question tests understanding of renal physiology, specifically the concepts of glomerular filtration rate (GFR), tubular maximum reabsorption rate (Tm), and the handling of substances by the renal tubules at different plasma concentrations.

## **Why the Correct Answer is Right**
At plasma concentrations above the Tm for a reabsorbed substance, the renal tubules are saturated with respect to reabsorption. This means that the maximum rate of reabsorption (Tm) is achieved and cannot increase further with higher plasma concentrations. The amount of substance excreted in the urine per unit time (excretion rate) equals the filtration rate (GFR * P) minus the reabsorption rate. Since reabsorption is at its maximum (Tm), the excretion rate is given by: Excretion rate = GFR * P - Tm. This directly corresponds to option .

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests that the excretion rate is directly proportional to the plasma concentration without considering the Tm, which is not applicable when the plasma concentration is above the Tm for reabsorption.
- **Option B:** This option incorrectly represents the relationship between excretion, GFR, P, and Tm. It inaccurately suggests a direct proportionality without accounting for the saturation of reabsorption.
- **Option D:** This option implies that there is no reabsorption occurring, which is incorrect. Even at high plasma concentrations, reabsorption occurs at the maximum rate (Tm).

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for substances with a high Tm (like glucose), their reabsorption is saturated at normal or slightly elevated plasma concentrations, leading to their appearance in urine (glycosuria) when plasma levels exceed the renal threshold (e.g., in diabetes mellitus).

## **Correct Answer:** .