**Core Concept**
The glomerular filtration rate (GFR) is a measure of the rate at which the kidneys filter the blood to produce urine. It is an essential indicator of renal function and is influenced by factors such as blood pressure, renal blood flow, and the permeability of the glomerular capillaries.

**Why the Correct Answer is Right**
The normal GFR in an adult human is approximately 125 milliliters per minute (ml/min) per 1.73 square meters of body surface area. This value is based on the glomerular filtration rate calculation, which takes into account the renal plasma flow and the filtration fraction. The glomerular filtration rate is a critical parameter for assessing kidney function and is often used to diagnose and monitor kidney diseases. The kidneys receive approximately 20-25% of the cardiac output, and the GFR is directly proportional to the renal plasma flow.

**Why Each Wrong Option is Incorrect**
**Option A:** A GFR of 50 ml/min is significantly lower than the normal value and may indicate severe kidney dysfunction or disease.
**Option C:** A GFR of 250 ml/min is higher than the normal value and may be seen in individuals with a large body surface area or those who are extremely athletic.
**Option D:** A GFR of 500 ml/min is excessively high and is not a physiologically normal value.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the GFR can be affected by various factors, including age, sex, and body size. The Cockcroft-Gault equation is often used to calculate the GFR in clinical practice, taking into account these factors.

**✓ Correct Answer: B. 125 ml/min**