Formation of the bony callus in fracture repair is followed by:

Choice A reason: Myosin is the thick filament in muscle fibers and is not directly attached to the Z disc. It anchors in the center of the sarcomere at the M line and interacts with actin during contraction.

Choice B reason: Actin is the correct answer. It forms the thin filaments in muscle fibers and is directly anchored to the Z disc. This attachment allows actin to slide past myosin during muscle contraction, shortening the sarcomere.

Choice C reason: Troponin is a regulatory protein complex that binds calcium and controls the position of tropomyosin on actin filaments. It is located along the actin filament but is not attached to the Z disc.

Choice D reason: Myofibrils are bundles of actin and myosin filaments organized into sarcomeres. While they span the length of the muscle fiber and include Z discs, they are not themselves attached to the Z disc.

Choice E reason: Tropomyosin is a regulatory protein that lies along the actin filament and blocks myosin-binding sites in a relaxed muscle. It is not anchored to the Z disc.

Choice A reason: A meatus is a canal-like passageway through bone, such as the external auditory meatus in the skull. It does not describe a rounded projection.

Choice B reason: A foramen is a hole or opening in a bone that allows the passage of nerves and blood vessels. It is not a projection.

Choice C reason: A condyle is a rounded articular projection that typically forms part of a joint. It allows for smooth articulation with another bone, making it the correct answer.

Choice D reason: A fossa is a shallow depression in a bone, often serving as an articular surface or for muscle attachment. It is not a projection.