## **Core Concept**
Normal male fertility requires a sufficient sperm count, with a threshold below which sterility or reduced fertility may occur. Sperm count is a critical factor in male fertility, and values are typically measured in millions per milliliter (mL) of ejaculate. The World Health Organization (WHO) provides reference values for semen characteristics.

## **Why the Correct Answer is Right**
The correct threshold for sterility in men, based on WHO criteria, is when the sperm count falls below 15 million per milliliter (mL) of ejaculate. This value is crucial because counts below this are associated with reduced fertility or sterility. Sperm count is just one aspect of semen analysis; other factors like motility and morphology also play significant roles in fertility.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 10 million - This is below the threshold but not the correct threshold value for defining sterility.
- **Option B:** 20 million - This is actually above the threshold for sterility; values at or above this are generally considered within the normal range for fertility.
- **Option D:** 5 million - This is significantly below the threshold and represents a more severe reduction in sperm count than the defined threshold for sterility.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while a sperm count below 15 million/mL is considered a factor in male infertility, other semen parameters like motility (<40% with progressive motility or <5% with rapid progressive motility) and morphology (<4% with normal forms) also significantly impact fertility. A comprehensive semen analysis is essential for evaluating male fertility. ## **Correct Answer:** . 15 million.