Articular cartilage found at the ends of long bones serves to:

Choice A reason: Pronation refers to the rotation of the forearm so that the palm faces downward. It does not describe movement away from the midline.

Choice B reason: Protraction refers to the anterior movement of a body part, such as pushing the shoulders forward. It does not involve lateral movement of the arm.

Choice C reason: This is correct. Abduction is the movement of a limb away from the midline of the body. Raising the arm sideways away from the torso is a classic example of abduction.

Choice D reason: Adduction is the opposite of abduction. It involves moving a limb toward the midline of the body.

Choice E reason: Retraction refers to the posterior movement of a body part, such as pulling the shoulders backward. It does not describe lateral arm movement.

Choice A reason: Actin is a structural protein that forms the thin filaments in muscle fibers. While it interacts with myosin during contraction, it does not bind calcium directly.

Choice B reason: Tropomyosin is a regulatory protein that blocks the myosin-binding sites on actin in a resting muscle. It shifts position when calcium binds to troponin but does not itself bind calcium.

Choice C reason: Titin is a large elastic protein that helps maintain the structural integrity of the sarcomere and contributes to passive elasticity. It does not function as a calcium receptor.

Choice D reason: Troponin is the correct answer. It is a regulatory protein complex associated with the thin filament. When calcium ions bind to troponin, it induces a conformational change that moves tropomyosin away from actin’s binding sites, allowing muscle contraction to occur.

Choice E reason: Dystrophin is a structural protein that connects the cytoskeleton of muscle fibers to the extracellular matrix. It is important for muscle integrity but does not bind calcium.