## Core Concept
The kidney contains two main types of nephrons based on their location and structure: cortical nephrons and juxtamedullary nephrons. Juxtamedullary nephrons are characterized by their location near the boundary between the cortex and medulla of the kidney, with their glomeruli situated near the corticomedullary junction.

## Why the Correct Answer is Right
Juxtamedullary nephrons constitute approximately 20-30% of the total nephrons in the human kidney. These nephrons play a crucial role in concentrating urine due to their long loops of Henle that extend deep into the renal medulla. This allows for efficient water reabsorption and concentration of the urine.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it suggests a much higher percentage than the known value for juxtamedullary nephrons.
* **Option B:** This option is incorrect as it represents a range but does not accurately reflect the commonly cited percentage for juxtamedullary nephrons.
* **Option D:** This option is incorrect because it suggests a much lower percentage than the established value.

## Clinical Pearl / High-Yield Fact
A key point to remember is that juxtamedullary nephrons are crucial for the kidney's ability to concentrate urine. Their long loops of Henle allow for a significant reabsorption of water back into the bloodstream, which is vital for maintaining proper fluid balance and electrolyte concentrations in the body.

## Correct Answer: C. 20-30%.