**Core Concept**
Basal Metabolic Rate (BMR) is the number of calories the body needs to function at rest. It is influenced by various factors, including age, sex, and body composition. The concept being tested here is the relationship between BMR and body composition, specifically lean body mass.

**Why the Correct Answer is Right**
Lean body mass (LBM) is the primary determinant of BMR. This is because LBM includes all the metabolically active tissues, such as muscles, organs, and bones, which require energy to maintain basic cellular functions. A higher LBM is associated with a higher BMR, as these tissues require more energy to function. The Harris-Benedict equation, a widely used formula to estimate BMR, takes into account LBM as a primary variable. In contrast, body mass index (BMI) and body surface area (BSA) are not as closely related to BMR, as they do not accurately reflect the metabolically active tissues.

**Why Each Wrong Option is Incorrect**
**Option B:** Body mass index (BMI) is a measure of body fat based on height and weight, but it does not accurately reflect the metabolically active tissues that contribute to BMR. BMI can be influenced by various factors, including muscle mass and bone density, making it a less reliable predictor of BMR.
**Option C:** Obesity is a state of excess body fat, but it does not directly influence BMR. While obesity can lead to changes in metabolism, the primary driver of BMR is still lean body mass.
**Option D:** Body surface area (BSA) is a measure of the body's surface area, but it is not a direct predictor of BMR. BSA can be influenced by various factors, including age, sex, and body composition, making it a less reliable predictor of BMR.

**Clinical Pearl / High-Yield Fact**
When estimating BMR, it is essential to use a formula that accurately reflects lean body mass, such as the Harris-Benedict equation. This is because LBM is the primary determinant of BMR, and using a formula that takes into account LBM will provide a more accurate estimate of resting energy expenditure.

**✓ Correct Answer: A. Lean body mass**