**Core Concept**
Net Protein Utilization (NPU) is a measure of the proportion of absorbed nitrogen from a protein source that is retained by the body and incorporated into proteins. It is an indicator of the quality of dietary protein, taking into account factors such as digestibility and bioavailability.

**Why the Correct Answer is Right**
NPU is calculated by dividing the amount of nitrogen retained by the amount of nitrogen consumed. This ratio is then multiplied by 6.25 to convert it to a percentage, representing the proportion of consumed protein that is utilized by the body. NPU values range from 0 to 1, with higher values indicating better protein quality. For example, high-quality proteins like whey have an NPU of 0.9, while low-quality proteins like gelatin have an NPU of 0.5.

**Why Each Wrong Option is Incorrect**
**Option A:**
This statement is incorrect because NPU does not measure the amount of protein required for optimal health. Instead, it evaluates the efficiency of protein utilization.

**Option B:**
This statement is incorrect because NPU is not directly related to the caloric content of a protein source. While high-calorie foods may contain protein, NPU specifically assesses the quality of the protein.

**Option C:**
This statement is incorrect because NPU is not a measure of protein synthesis. While protein synthesis is an important aspect of protein metabolism, NPU focuses on the utilization of absorbed nitrogen.

**Clinical Pearl / High-Yield Fact**
When evaluating protein quality, consider the NPU as a key factor. A high NPU indicates that the protein is well-digested and efficiently utilized by the body. This is particularly important for patients with malabsorption or those requiring high-protein diets for medical conditions.

**Correct Answer: C. NPU measures the proportion of absorbed nitrogen from a protein source that is retained by the body and incorporated into proteins.**