Which of the following is an example of a sesamoid bone?

Choice A reason: Central lamellae do not exist as a recognized anatomical term. The central canal, however, is part of an osteon and contains blood vessels and nerves. It does not refer to lamellae between osteons.

Choice B reason: Circumferential lamellae are located around the outer and inner surfaces of compact bone, encircling the entire bone structure. They are not found between osteons.

Choice C reason: Interstitial lamellae are remnants of old osteons that have been partially resorbed during bone remodeling. They fill the spaces between current osteons and are not part of any single osteon, making them the correct answer.

Choice D reason: Concentric lamellae are rings of bone matrix that surround the central canal within an osteon. They are integral to the osteon structure and not found between osteons.

Choice A reason: Myosin heads pulling actin toward the center of the sarcomere is the core mechanism of cross-bridge cycling. This interaction shortens the sarcomere and generates muscle contraction. ATP binding and hydrolysis drive the cycle of attachment, power stroke, and detachment.

Choice B reason: Thick filaments do not shorten during contraction. Instead, thin filaments slide past the thick filaments as the sarcomere shortens. The filaments themselves remain the same length.

Choice C reason: Actin and myosin do not lengthen during contraction. They maintain their structural integrity while sliding past each other through repeated cross-bridge interactions.

Choice D reason: Z discs define the boundaries of a sarcomere and move closer together during contraction, but they do not slide over myofilaments. Their movement is a result of filament sliding, not a direct action.

Choice E reason: Titin is a structural protein that contributes to passive elasticity and sarcomere stability. It does not actively shorten or participate in the cross-bridge cycle