Which of the following is NOT one of the major body systems?

The endocrine system is not typically classified as one of the major body systems when compared to the other options listed. This classification often emphasizes systems that play more direct or critical roles in immediate physiological functions and overall structural support essential for organismal survival.

The major body systems commonly recognized include the musculoskeletal system, which provides structure and enables movement; the respiratory system, responsible for gas exchange and maintaining oxygen levels in the blood; and the circulatory system, which transports nutrients, oxygen, and waste products throughout the body. While the endocrine system is indeed crucial for regulating many bodily functions, including metabolism, growth, and mood through hormone secretion, it is often considered a more specialized system rather than one of the foundational systems that support the structural and functional integrity of the body in the same way as the others listed.