**Core Concept**
Severe malnutrition in adults is a critical condition characterized by a significantly reduced body mass index (BMI), indicating inadequate nutrition and potential health risks. The World Health Organization (WHO) defines severe malnutrition as a BMI below a specific threshold.

**Why the Correct Answer is Right**
The correct answer is B. 16, as the WHO defines severe malnutrition in adults with a BMI less than 16. This is because a BMI below 16 indicates a severe loss of body fat and muscle mass, which can lead to serious health complications, including increased susceptibility to infections, impaired wound healing, and reduced immune function. In adults, a BMI of 16 or less is associated with a higher risk of mortality, making it a critical threshold for defining severe malnutrition.

**Why Each Wrong Option is Incorrect**
**Option A:** A BMI of 18.5 is actually the threshold for normal weight according to the WHO, not severe malnutrition.
**Option C:** A BMI of 13 is extremely low and may indicate a life-threatening condition, but it is not the threshold used by the WHO to define severe malnutrition.
**Option D:** A BMI of 11 is also extremely low and is not the threshold used by the WHO to define severe malnutrition.

**Clinical Pearl / High-Yield Fact**
When assessing a patient's nutritional status, it is essential to consider not only their BMI but also other factors such as weight loss, muscle wasting, and edema. A low BMI alone may not accurately reflect a patient's nutritional status, and a comprehensive assessment should be conducted to determine the severity of malnutrition.

✓ Correct Answer: B. 16