Spongy bone contains:

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

Choice B reason: Tropomyosin is a regulatory protein that blocks the myosin-binding sites on actin in a relaxed muscle. It shifts position when calcium binds to troponin, but it 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 that binds calcium ions during muscle contraction. This binding causes a conformational change that moves tropomyosin away from actin’s binding sites, allowing myosin to interact with actin and initiate contraction.

Choice E reason: Dystrophin is a structural protein that connects the cytoskeleton of muscle fibers to the extracellular matrix. It plays a role in muscle integrity but does not bind calcium or regulate contraction.

Choice A reason: Bipennate muscles have fibers arranged on both sides of a central tendon, resembling a feather. They are not spindle-shaped and do not have parallel fibers.

Choice B reason: Fusiform muscles are spindle-shaped with parallel fibers that taper at both ends. This structure allows for efficient force generation along the length of the muscle, such as in the biceps brachii.

Choice C reason: Multipennate muscles have multiple rows of fibers attached to a central tendon. They are not spindle-shaped and are typically found in muscles requiring high force output, like the deltoid.

Choice D reason: Circular muscles have fibers arranged in concentric rings, typically found around openings such as the mouth or eyes. They are not spindle-shaped and do not have parallel fibers.