Loss of muscle mass from lack of activity is termed:

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.

Choice A reason: Synarthrotic joints are structurally designed to be immobile, often found in areas like the skull where bones are tightly joined by fibrous tissue. Because of their rigidity and lack of movement, they are not prone to dislocation.

Choice B reason: Freely movable joints are classified as diarthroses, such as the shoulder or knee. Synarthrotic joints are the opposite, offering no movement between the bones.

Choice C reason: Slightly movable joints are called amphiarthroses, such as the joints between vertebrae. Synarthrotic joints do not allow even slight movement.

Choice D reason: Synarthrotic joints are immovable joints where bones are connected by fibrous tissue or cartilage. Examples include sutures in the skull. Their primary function is protection and structural stability.