## **Core Concept**
The absolute neutrophil count (ANC) is a critical measure used to assess the risk of infection in patients, particularly those with neutropenia. Neutropenia is a condition characterized by an abnormally low number of neutrophils, which are a type of white blood cell crucial for fighting infections. The ANC is calculated by multiplying the total white blood cell count by the percentage of neutrophils.

## **Why the Correct Answer is Right**
A significant decrease in ANC increases the risk of serious infections because neutrophils play a key role in combating bacterial and fungal infections. When the ANC drops below 1,000 cells/μL, the risk of infection starts to increase, and at levels below 500 cells/μL, the risk becomes significantly higher. Therefore, an ANC of less than 500 cells/μL is generally considered a critical threshold below which serious infections can occur.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 5,000 cells/μL is within the normal range for ANC (1,500-8,000 cells/μL), and at this level, the risk of infection due to neutropenia is not significantly increased.
- **Option B:** 1,000 cells/μL is a threshold below which the risk of infection starts to increase, but it is not typically considered the critical level at which serious infections are likely to occur.
- **Option D:** 2,000 cells/μL is also within a range where some risk exists but is not the specific threshold for a significantly increased risk of serious infections.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with an ANC below 500 cells/μL are considered to have severe neutropenia and are at a significantly increased risk of life-threatening infections. This condition often necessitates prompt medical intervention, including the initiation of antibiotics.

## **Correct Answer:** . 500 cells/μL.