Define "lean body mass."

Lean body mass refers to the total weight of everything in the body except fat. This includes not only muscles but also bones, organs, blood, and water. Understanding lean body mass is crucial in assessing an individual’s body composition, as it gives insight into how much of the body's weight is made up of tissues that are metabolically active. In contrast, options that specify only muscles or limit the definition to organs and tissues do not encompass the full range of components that contribute to lean body mass. Additionally, defining lean body mass in terms of fat weight alone inaccurately defines it by what it lacks rather than what it includes. Therefore, the choice that encompasses all non-fat tissues correctly defines lean body mass.