**Core Concept**
The renal threshold for glucose is the serum glucose concentration at which glucose begins to appear in the urine (glycosuria). This threshold is determined by the maximum reabsorption capacity of the proximal convoluted tubule via the sodium-glucose cotransporter 2 (SGLT2) system. When serum glucose exceeds this level, the renal tubules cannot reabsorb all glucose, leading to glucose excretion in urine.

**Why the Correct Answer is Right**
The renal threshold for glucose is approximately **180–200 mg/dL**, with **200 mg/dL** being the standard value cited in clinical physiology. At this level, the reabsorption capacity of the proximal tubule is saturated, and excess glucose spills into the filtrate and is excreted in urine. This threshold is a key concept in diagnosing diabetes mellitus, as persistent glycosuria beyond this level indicates hyperglycemia.

**Why Each Wrong Option is Incorrect**
Option A: 100 mg/dL – This is below the renal threshold; glucose is fully reabsorbed at this level and no glycosuria occurs.
Option C: 300 mg/dL – This is above the threshold, but not the standard reference point; it is a higher value seen in severe hyperglycemia, not the renal threshold.
Option D: 400 mg/dL – This is a level seen in uncontrolled diabetes, but it is not the threshold; it reflects ongoing hyperglycemia, not the point of spillage.

**Clinical Pearl / High-Yield Fact**
Remember: **Glucose appears in urine when serum glucose exceeds 200 mg/dL** — this is the renal threshold. If glucose is detected in urine, it's a sign of hyperglycemia, especially if sustained.

✓ Correct Answer: B. 200 mg/dl