**Core Concept**
The renal threshold for glycosuria is the plasma glucose concentration above which glucose begins to appear in the urine. This concept is crucial in understanding the relationship between blood glucose levels and urinary glucose excretion.

**Why the Correct Answer is Right**
The renal threshold for glycosuria is approximately 180 mg/dl. This is because at this concentration, the reabsorption capacity of the renal proximal tubules is exceeded, and glucose starts to be filtered into the urine. The glucose transporters in the renal tubules, particularly SGLT2, play a key role in reabsorbing glucose back into the bloodstream. However, when the plasma glucose concentration exceeds 180 mg/dl, the reabsorption capacity of SGLT2 is overwhelmed, leading to the appearance of glucose in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** 100 mg/dl is below the renal threshold for glycosuria, as glucose is still being reabsorbed by the renal tubules at this concentration. **Option C:** 300 mg/dl is higher than the renal threshold, but glucose is still being reabsorbed in the early proximal tubules. **Option D:** 350 mg/dl is also higher than the renal threshold, but glucose is still being reabsorbed in the late proximal tubules.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the renal threshold for glycosuria is a dynamic concept and can vary depending on factors such as hydration status, kidney function, and the presence of underlying metabolic disorders. A fasting plasma glucose level of 180 mg/dl is typically considered the upper limit of normal for glucose tolerance.

✓ Correct Answer: B. 180 mg/dl