The _____ is located on the _____ bone and is responsible for protecting the pituitary gland.

Choice A reason: Synarthrotic joints are immovable, not slightly movable. Examples include sutures in the skull. They are designed for stability rather than movement.

Choice B reason: Synovial joints are the only type of joint that is always freely movable. They include hinge joints (e.g., elbow), ball-and-socket joints (e.g., shoulder), and others that allow a wide range of motion due to the presence of a synovial cavity and lubricating fluid.

Choice C reason: A gomphosis is a type of fibrous joint found between teeth and their sockets. It is immovable and not freely movable.

Choice D reason: Cartilaginous joints do not have a joint cavity. They are connected entirely by cartilage and allow limited movement, such as the intervertebral discs and the pubic symphysis.

Choice E reason: Immovable joints are called synarthroses, not amphiarthroses. Amphiarthroses are slightly movable joints, such as those between vertebrae.

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.