**Core Concept**
The Glomerular Filtration Rate (GFR) is a crucial indicator of kidney function, representing the volume of fluid filtered from the renal glomeruli per unit time. It is calculated using the formula: GFR = (Urine creatinine × Serum creatinine) / (Urine specific gravity × 135). The denominator in the GFR formula is crucial for accurate calculation.

**Why the Correct Answer is Right**
The denominator in the GFR formula is 135, which is a constant representing the average plasma concentration of creatinine in a healthy person. This value is used as a reference to calculate the GFR. The constant 135 is derived from the average plasma creatinine concentration in a healthy adult, which is approximately 1.3 mg/dL. When the serum creatinine value is known, the GFR can be calculated using the formula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because there is no scientific basis for using 100 as the denominator in the GFR formula.
**Option B:** This option is incorrect because 150 is not a standard value used in the GFR formula. The correct value is 135, which is a well-established reference for plasma creatinine concentration.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the GFR formula uses a constant denominator to standardize the calculation, allowing for accurate comparison of kidney function across different individuals.

**Correct Answer: C. 135**