**Core Concept**
Urea clearance is a measure of the kidneys' ability to remove waste products from the blood. It is an important indicator of renal function, with standard values reflecting the average efficiency of the kidneys in filtering and excreting urea.

**Why the Correct Answer is Right**
The standard urea clearance in a normal adult is approximately 50-60 mL/min. This value is derived from the glomerular filtration rate (GFR), which is the rate at which the kidneys filter the blood to remove waste products. The GFR is influenced by the number of functioning nephrons, the surface area of the glomeruli, and the hydrostatic pressure within the glomeruli. In a normal adult, the GFR is approximately 125 mL/min, which corresponds to a urea clearance of 50-60 mL/min.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely to be incorrect as urea clearance is not directly related to the creatinine clearance.
* **Option B:** This option may be incorrect as the urea clearance in a normal adult is not typically reported as 20 mL/min.
* **Option D:** This option may be incorrect as the urea clearance in a normal adult is not typically reported as 100 mL/min.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that urea clearance can be affected by various factors, including age, sex, body size, and renal function. In patients with impaired renal function, urea clearance may be reduced, making it a useful marker for assessing kidney function.

**Correct Answer: C. 50-60 mL/min**