## Core Concept
The human kidney is a complex organ responsible for filtration of blood, regulation of electrolytes, and maintenance of acid-base balance. It achieves these functions through the nephron, the functional unit of the kidney. Each nephron is a microscopic structure that filters waste and excess substances from the blood.

## Why the Correct Answer is Right
The correct answer states that each kidney contains about 1-2 million nephrons, but more accurately, it is approximately **1 million nephrons per kidney**. Nephrons are responsible for the filtration of blood and are composed of several parts including the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The large number of nephrons allows for efficient filtration and regulation of blood composition.

## Why Each Wrong Option is Incorrect
- **Option A:** 1000 nephrons - This is far too low considering the functional capacity of the kidney and the actual number of nephrons present.
- **Option B:** 100 nephrons - Similarly, this is an underestimation and does not accurately reflect the nephron count.
- **Option D:** 10 - This is an extremely low number and not representative of the nephron count in human kidneys.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while the average number of nephrons per kidney is about 1 million, there can be significant variability among individuals, and this number does not regenerate if nephrons are lost due to disease. Nephron loss can lead to chronic kidney disease or end-stage renal disease.

## Correct Answer: C. 1 million.