A substance is present in a concentration of 2mg% in the afferent arteriole and zero mg% in the efferent. True about the substance is
**Core Concept**
The question is testing the concept of tubular reabsorption and secretion in the kidneys. The substance present in the afferent arteriole and its absence in the efferent arteriole suggests that it is being reabsorbed by the renal tubules.
**Why the Correct Answer is Right**
The correct answer is related to the reabsorption of glucose by the renal tubules. The renal tubules reabsorb glucose from the filtrate back into the bloodstream, which is essential for maintaining glucose homeostasis. The presence of glucose in the afferent arteriole and its absence in the efferent arteriole indicates that it is being reabsorbed by the renal tubules. This process is mediated by sodium-glucose cotransporters (SGLTs) in the proximal convoluted tubule.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the substance is likely glucose, which is reabsorbed by the renal tubules, not secreted.
* **Option B:** This option is incorrect because the substance is reabsorbed, not filtered out by the kidneys.
* **Option C:** This option is incorrect because the substance is glucose, which is not a waste product that is concentrated in the urine.
* **Option D:** This option is incorrect because the substance is reabsorbed, not excreted by the kidneys.
**Clinical Pearl / High-Yield Fact**
Glucose reabsorption by the renal tubules is a critical function that helps maintain glucose homeostasis. However, in cases of hyperglycemia, the renal tubules can become saturated with glucose, leading to glucose excretion in the urine (glycosuria).
**Correct Answer: C. Glucose.