**Core Concept**
Body Metabolic Rate (BMR) is the rate at which the body burns energy to maintain basic physiological functions. It is influenced by various factors, including genetics, age, sex, weight, and body composition.

**Why the Correct Answer is Right**
BMR is primarily dependent on the amount of lean body mass, particularly muscle mass. Muscle tissue requires more energy to maintain than fat tissue, and the more muscle mass an individual has, the higher their BMR will be. This is due to the metabolic activity of muscle cells, which require energy to maintain their structural and functional integrity.

**Why Each Wrong Option is Incorrect**
**Option A:** Age is a factor that affects BMR, but it is not the primary determinant. While BMR does decrease with age, other factors such as muscle mass and body composition play a more significant role.

**Option B:** Sex is also a factor that influences BMR, with men generally having a higher BMR than women due to their higher muscle mass. However, it is not the primary determinant of BMR.

**Option C:** Body weight is a factor that affects BMR, but it is not the primary determinant. BMR is more closely related to lean body mass than to overall body weight.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that a decrease in BMR can be an indicator of certain medical conditions, such as hypothyroidism or cachexia. In these cases, BMR may be lower than expected for the individual's age, sex, and weight.

**Correct Answer:** C. Body weight