This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Most commonly used for protein utilization ? 
Select and explain. 
A. NPU, B. Amino acid score, C. Biological value, D. Protein efficiency ratio
A: NPU
Answer: A: NPU 
Explanation: Ans. is `a' i.e., NPU