The function of the axial skeleton includes:

Correct Answer: C

Rationale: The axial skeleton, comprising the skull, vertebral column, and rib cage, primarily serves to protect vital organs. The skull safeguards the brain, the vertebral column shields the spinal cord, and the rib cage encases the heart and lungs. While it does provide some support and attachment for muscles, its defining role is protection, distinguishing it from the appendicular skeleton, which is more involved in locomotion. Thus, 'Protecting vital organs' is the most accurate answer.

Correct Answer: A

Rationale: In the sarcomere, myosin heads bind to actin filaments during contraction, as per the sliding filament theory. This interaction pulls actin toward the sarcomere center, shortening it. Troponin and tropomyosin regulate access to actin’s binding sites, while collagen is not involved, making 'Actin' the correct answer.

Correct Answer: C

Rationale: Sesamoid bones, like the patella, are embedded within tendons and primarily enhance muscle leverage. The patella improves the mechanical advantage of the quadriceps muscle, increasing force during knee extension. While they offer some protection, their key role is leverage, making 'Enhancing muscle leverage' the correct answer.

Correct Answer: C

Rationale: The pelvic girdle, part of the appendicular skeleton, supports the lower limbs (via hip joints) and protects pelvic organs (e.g., bladder). It does not protect the spinal cord (vertebral column’s role) or aid respiration, making 'Supporting the lower limbs and organs' the correct answer.

Correct Answer: B

Rationale: Troponin, a protein on actin filaments, binds calcium ions released from the sarcoplasmic reticulum. This binding shifts tropomyosin, exposing actin’s myosin-binding sites, enabling contraction. Tropomyosin prevents binding in a relaxed state, making 'Binding to calcium ions' the correct answer.