What is the role of troponin in muscle contraction?

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.

Correct Answer: C

Rationale: The ribs, part of the thoracic cage in the axial skeleton, form a protective enclosure around vital organs. The 12 pairs of ribs, along with the sternum and thoracic vertebrae, shield the heart and lungs directly. The liver, located below the diaphragm, is partially protected by the lower ribs. Vertebrae protect the spinal cord, the skull protects the brain, and the femur is a leg bone, making 'ribs' the correct answer.

Correct Answer: B

Rationale: The skeleton protects soft organs (e.g., skull protects brain), supports the body (via structure), and stores minerals (calcium, phosphorus) and fat (yellow marrow). Vitamin D production occurs in the skin upon sunlight exposure, not the skeleton, though bones use vitamin D to absorb calcium. Thus, 'produces vitamin D' is NOT a skeletal function.

Correct Answer: A

Rationale: The epiphyses of long bones are covered with articular (hyaline) cartilage at joints, reducing friction and protecting bone ends during movement. Red and yellow marrow are inside bones, and the endosteum lines the medullary cavity, making 'articular (hyaline) cartilage' correct.

Correct Answer: C

Rationale: Perforating canals (Volkmann’s canals) connect central canals (Haversian canals) in osteons, allowing blood vessels to travel between them. Canaliculi connect osteocytes, the medullary canal is the marrow cavity, and trabeculae are in spongy bone, making 'perforating canal' correct.