**Core Concept**
Biological value (BV) of a protein measures the efficiency with which the body utilizes protein to build and repair tissues. It is primarily determined by the amount of nitrogen absorbed and utilized, as nitrogen is a key component of amino acids and is essential for protein synthesis.

**Why the Correct Answer is Right**
The biological value is calculated based on the nitrogen content because nitrogen is a reliable indicator of protein quality. When a protein is ingested, the body absorbs nitrogen, and the amount absorbed over time reflects how well the protein is utilized. The nitrogen content is measured via the Kjeldahl method, and the BV is derived from the ratio of absorbed nitrogen to the nitrogen in the protein. Higher nitrogen retention indicates higher biological value.

**Why Each Wrong Option is Incorrect**
Option B: Amino acid content is important for protein quality and completeness, but biological value is not directly calculated from amino acid composition. It is nitrogen content that serves as the basis for BV calculation.
Option C: Sulphur content is relevant to specific amino acids like cysteine and methionine, but it does not determine overall biological value.
Option D: Energy content refers to calories, which is not used in calculating biological value; proteins are assessed by their nitrogen utilization, not energy yield.

**Clinical Pearl / High-Yield Fact**
The biological value is always calculated using nitrogen content, and a high BV (e.g., >100) indicates a protein is efficiently utilized by the body. This concept is vital in nutritional assessment and planning for patients with malnutrition or chronic diseases.

✓ Correct Answer: A. Nitrogen content