**Core Concept**
Glucosuria is the presence of glucose in the urine, which occurs due to the kidneys' inability to reabsorb glucose from the filtrate. Normally, the kidneys reabsorb glucose back into the bloodstream when the blood glucose concentration is within a certain range. However, when the blood glucose concentration exceeds this range, the kidneys cannot reabsorb all the glucose, leading to its excretion in the urine.

**Why the Correct Answer is Right**
When the blood glucose concentration exceeds 180 mg/dL (10 mmol/L), the kidneys' reabsorptive capacity for glucose is overwhelmed, and glucose begins to appear in the urine. This is because the renal threshold for glucose is approximately 180 mg/dL, above which the kidneys cannot reabsorb glucose efficiently. The glucose molecule is then filtered out and excreted in the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** The renal threshold for glucose is not 140 mg/dL, as this is below the normal reabsorption range.
* **Option B:** The renal threshold for glucose is not 220 mg/dL, as this is above the typical range for glucosuria to occur.
* **Option D:** The renal threshold for glucose is not 250 mg/dL, as this is significantly higher than the established threshold.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the renal threshold for glucose can be altered in certain conditions, such as diabetes mellitus, where the kidneys' ability to reabsorb glucose is impaired. This can lead to glucosuria even at lower blood glucose concentrations.

**Correct Answer: C. 180 mg/dL**