**Core Concept**
The renal threshold for glucose is the plasma glucose concentration at which the kidneys begin to reabsorb glucose back into the bloodstream rather than excreting it in the urine. This concept is crucial in understanding glucose homeostasis and the pathophysiology of diabetes mellitus.

**Why the Correct Answer is Right**
The renal threshold for glucose is approximately 180 mg/dl. This is because the renal tubules have a limited capacity to reabsorb glucose, and when this capacity is exceeded, glucose starts to appear in the urine. The glucose transporters (SGLT2) in the proximal convoluted tubule (PCT) play a key role in glucose reabsorption. Normally, these transporters reabsorb glucose from the filtrate back into the bloodstream, but when plasma glucose levels exceed 180 mg/dl, the kidneys are overwhelmed, and glucose starts to spill into the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** 80 mg% is too low to be the renal threshold for glucose, as it is below the normal fasting glucose range.
**Option B:** 100 mg% is still below the renal threshold for glucose, as the kidneys can reabsorb glucose from the filtrate at this concentration.
**Option D:** 200 mg% is above the renal threshold, but it is not the specific value at which glucose starts to appear in the urine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the renal threshold for glucose is not fixed and can be influenced by various factors, such as age, kidney function, and certain medications. In patients with diabetes mellitus, the renal threshold for glucose can be elevated due to prolonged exposure to high glucose levels, leading to glycosuria.

**✓ Correct Answer: C. 180 mg/dl**