**Question:** Protein quality assessment is best done by –

A. Total nitrogen content
B. EiYaji test
C. Biological value
D. Protein efficiency ratio

**Core Concept:** Assessing the quality of proteins involves evaluating their ability to be utilized by the body for growth, maintenance, and repair. Protein quality is determined by considering factors such as amino acid composition, digestibility, and biological value.

**Why the Correct Answer is Right:** The correct answer, **D. Protein Efficiency Ratio (PER)**, reflects the ratio of body weight gain and feed intake in animals when fed with test protein and reference protein. It indicates the efficiency with which protein is utilized by the body and is a good indicator of protein quality.

**Why Each Wrong Option is Incorrect:**

A. Total nitrogen content (Option A) is an indicator of the amount of nitrogen in protein, but it does not account for the amino acid composition or digestibility, which are crucial for protein quality.

B. EiYaji test (Option B) is a simplified method to assess protein quality that uses rats, but it is not widely accepted and has limitations in reflecting human protein utilization.

C. Biological value (Option C) measures the percentage of nitrogen absorbed and utilized by the body, but it is based on human trials, which can be time-consuming and expensive.

**Clinical Pearl / High-Yield Fact:** Protein quality assessment is essential for determining the nutritional value of different protein sources. For example, in a diet, consuming a low-quality protein alongside a high-quality protein can result in a protein with an intermediate quality score. However, proteins with high digestibility and amino acid profile are considered superior. Understanding protein quality assessment aids in formulating balanced diets, especially in the context of infant formula, sports nutrition, and clinical nutrition support.