Loss of muscle mass from lack of activity is termed:

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: While the femur is a large and strong bone, its size alone does not contribute significantly to joint stability. Stability is more dependent on the structural configuration of the joint and surrounding tissues.

Choice B reason: Muscles around the hip joint, such as the gluteals and iliopsoas, contribute to movement and dynamic stability, but they are not the primary factor in maintaining joint integrity under load.

Choice C reason: Articular cartilage provides a smooth surface for joint movement and helps reduce friction, but it does not significantly enhance joint stability. Its role is more about cushioning and facilitating motion.

Choice D reason: The depth of the acetabulum is the most critical factor in hip joint stability. This deep socket securely houses the head of the femur, forming a ball-and-socket joint that resists dislocation and supports weight-bearing activities. The acetabular labrum further deepens the socket and enhances stability.