## **Core Concept**
The absolute neutrophil count (ANC) is a critical measure used to assess the body's ability to fight infections, particularly those caused by bacteria and fungi. Neutrophils are a type of white blood cell that plays a key role in the innate immune response. A severely low ANC, a condition known as neutropenia, increases the risk of severe infections.

## **Why the Correct Answer is Right**
The correct answer indicates that an ANC of less than 500 cells/μL significantly increases the risk of severe infections. This is because neutrophils are crucial for phagocytosing and killing pathogens. When their count drops below this threshold, the body's ability to fight off infections is severely compromised. Patients with such low ANC are considered to be at a high risk of developing infections that can be life-threatening if not promptly treated.

## **Why Each Wrong Option is Incorrect**
- **Option A:** An ANC of 1500 cells/μL is within the normal range (typically 1500-8000 cells/μL) and does not indicate an increased risk of severe infections due to neutropenia.
- **Option B:** An ANC of 1000 cells/μL is considered mild neutropenia and while it does indicate some risk, it's not as high as with more severe neutropenia.
- **Option D:** An ANC of 2000 cells/μL is also within a range that might be considered mild to moderate neutropenia but is not indicative of the highest risk for severe infections as defined by the most critical threshold.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of infection increases significantly when the ANC falls below 500 cells/μL. This is a critical threshold used in clinical practice to guide the management of patients with neutropenia, including the initiation of antimicrobial therapy.

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