Which of the following bones is part of the axial skeleton?

Correct Answer: D

Rationale: The sternum (breastbone) is part of the axial skeleton, forming the thoracic cage with the ribs to protect the heart and lungs. Scapula (shoulder blade), humerus (upper arm), and femur (thigh) are appendicular skeleton bones, involved in limb movement, not axial support.

Correct Answer: C

Rationale: Bones are classified into four main types based on shape: long, short, flat, and irregular. Long bones (e.g., femur) are longer than they are wide, short bones (e.g., carpals) are roughly equal in length and width, and flat bones (e.g., scapula) are thin and broad. Irregular bones, such as the vertebrae or certain skull bones, have complex shapes that do not fit neatly into the other categories due to their unique structure and function, making 'Irregular bone' the correct answer.

Correct Answer: A

Rationale: In muscle contraction, calcium ions play a key role in exposing binding sites on actin filaments. Troponin, a regulatory protein on the actin filament, binds to calcium ions when they are released from the sarcoplasmic reticulum. This binding causes tropomyosin to shift, uncovering myosin-binding sites on actin, allowing myosin heads to attach and initiate contraction. Thus, 'Troponin' is the correct answer.

Correct Answer: C

Rationale: Muscle contraction begins when calcium ions (Ca2+) are released from the sarcoplasmic reticulum into the muscle cell cytoplasm. Calcium binds to troponin, shifting tropomyosin to expose actin binding sites, allowing myosin to attach and contract the muscle. Sodium and potassium regulate nerve signals, not direct contraction, making 'Calcium (Ca2+)' the correct answer.

Correct Answer: D

Rationale: The scapula is a flat bone, characterized by its thin, broad shape. Flat bones provide protection and muscle attachment surfaces; the scapula protects the shoulder region and anchors upper limb muscles. Long bones are cylindrical, short bones are cube-like, and irregular bones have complex shapes, making 'Flat bone' the correct answer.