What factor closely relates to the basal metabolic rate (BMR) as a child grows?

• A. Proportion of surface area to body mass.
• B. Body mass alone.
• C. Hair color.
• D. Blood type.

Answer: (A)

The main idea here is how growth changes the body's energy needs in relation to surface area. Basal metabolic rate (BMR) is the amount of energy the body needs at rest to maintain basic functions, and part of that energy goes to regulating body temperature. Heat loss is tied to the body's surface area, so the proportion of surface area to body mass matters: more surface area per unit of mass means more heat loss and higher energy needs to compensate.

As a child grows, their body mass increases faster than their surface area (surface area grows roughly with length squared, while mass grows with length cubed), so the surface area-to-mass ratio decreases. This means BMR per kilogram tends to decline with age, even though the total BMR rises as the child gains more body tissue. Hair color or blood type don’t influence BMR in this growth-related way, and body mass alone doesn’t capture how changes in heat loss affect energy needs across growth.