Which of the following is the smallest unit of a muscle fiber that can contract?

Correct Answer: B

Rationale: A muscle fiber contains myofibrils, which are composed of repeating units called sarcomeres. The sarcomere is the smallest structural unit of a muscle that can contract, defined as the segment between two Z-lines, where actin and myosin filaments interact to produce contraction. Myosin and actin are proteins within the sarcomere, not contractile units themselves, making 'Sarcomere' the correct answer.

Correct Answer: C

Rationale: The axial skeleton includes the skull, vertebral column, and rib cage. The vertebral column, a series of vertebrae, forms a bony canal that encases and protects the spinal cord. The rib cage protects thoracic organs, the skull protects the brain, and the pelvic girdle is appendicular, making 'Vertebral column' the correct answer.

Correct Answer: A

Rationale: ATP (adenosine triphosphate) provides the energy for muscle contraction by binding to myosin heads, allowing them to detach from actin and re-cock for another pull. Glucose is a fuel source, oxygen supports aerobic respiration, and lactic acid is a byproduct, making 'ATP' the correct answer.

Correct Answer: A

Rationale: In muscle contraction, the sliding filament theory states that actin and myosin filaments slide past each other, shortening the sarcomere (the distance between Z-lines). This shortening contracts the muscle. It does not lengthen, stay unchanged, or vibrate, making 'It shortens' the correct answer.

Correct Answer: B

Rationale: Carpals and tarsals are short bones, roughly equal in length and width, providing stability and flexibility in wrist and ankle joints. They are not irregularly shaped (like vertebrae) but are classified as short, making 'Short bone' the correct answer despite the question’s phrasing.