**Core Concept**
Acute malnutrition is a critical condition characterized by a rapid decline in body weight and muscle mass, often resulting from inadequate nutrition, disease, or other stressors. The key to identifying acute malnutrition lies in assessing body composition and nutritional status.

**Why the Correct Answer is Right**
The mid-upper arm circumference (MUAC) is a widely accepted and effective indicator of acute malnutrition, particularly in children. MUAC measures the circumference of the mid-upper arm, which is a reliable proxy for muscle mass and overall nutritional status. This simple, non-invasive measurement is particularly useful in resource-constrained settings or when access to more advanced diagnostic tools is limited. The World Health Organization (WHO) recommends MUAC as a key indicator of acute malnutrition in children aged 6-59 months.

**Why Each Wrong Option is Incorrect**
**Option A:** Weight-for-height is a measure of acute malnutrition, but it is not as sensitive as MUAC, especially in children with a high muscle mass or those with a recent weight gain.
**Option B:** Height-for-age is a measure of chronic malnutrition, which reflects long-term growth failure and is not indicative of acute malnutrition.
**Option C:** Body mass index (BMI) is a measure of overweight and obesity, not acute malnutrition.

**Clinical Pearl / High-Yield Fact**
Remember that MUAC is a simple, effective, and non-invasive tool for identifying acute malnutrition, particularly in children. Always consider MUAC as a key component of nutritional assessment in pediatric patients.

**Correct Answer: C. Body mass index (BMI) is a measure of overweight and obesity, not acute malnutrition.**