## **Core Concept**
The Protein Efficiency Ratio (PER) is a method used to evaluate the quality of a protein in a diet. It is defined as the ratio of weight gain of the test subject (usually an animal) to the intake of a particular protein. This measure helps in understanding how efficiently a protein source supports growth.

## **Why the Correct Answer is Right**
The correct formula for Protein Efficiency Ratio (PER) is expressed as:
PER = Weight gain (g) / Protein intake (g).
This formula directly compares the gain in weight (which is an indicator of growth and thus the utilization of protein for growth) to the amount of protein consumed. It specifically focuses on the protein component of the diet, making it a valuable tool for assessing protein quality.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not represent a recognized formula for PER. Without the specific details of the formula, it's clear that it does not align with the standard definition of PER.
- **Option B:** Similarly, this option does not match the standard formula for calculating PER. It seems to introduce a factor (possibly related to calories or another nutrient) that is not part of the PER calculation.
- **Option D:** This option also does not correspond to the established method for calculating PER. It might confuse the concept with another nutritional ratio or index.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that PER is particularly useful in experimental settings to compare the quality of different protein sources. However, it has limitations, such as not accounting for the protein's digestibility or its content of essential amino acids. A high PER value indicates a high-quality protein that is efficiently used by the body for growth.

## **Correct Answer:** . Weight gain (g) / Protein intake (g)