Which of the following muscles is responsible for smiling?

Choice A reason: The platysma is a superficial neck muscle involved in depressing the mandible and tensing the skin of the neck. It is not associated with facial expressions like smiling.

Choice B reason: The masseter is a powerful jaw muscle responsible for chewing. It does not contribute to smiling or ceremonial expressions.

Choice C reason: The sternocleidomastoid is a neck muscle involved in head rotation and flexion. It is not used in facial expressions.

Choice D reason: The orbicularis oris encircles the mouth and is responsible for movements like puckering or closing the lips. While it may be involved in kissing, it is not the primary muscle used for smiling.

Choice E reason: The zygomaticus muscle, specifically the zygomaticus major, elevates the corners of the mouth and is the primary muscle used in smiling. It is commonly activated during joyful expressions, such as those seen in wedding ceremonies.

Choice A reason: While coordination is important in highly mobile joints, this is not the primary reason for the vulnerability of the glenohumeral joint.

Choice B reason: Ligaments around the shoulder joint are indeed more flexible to allow movement, but this alone does not fully explain the joint’s instability.

Choice C reason: Power generation is not significantly reduced due to mobility. Muscles around the shoulder can generate substantial force.

Choice D reason: Blood vessels are generally well protected and not easily compromised by joint movement. This is not a common consequence of shoulder mobility.

Choice E reason: This is the correct answer. The glenohumeral joint is a ball-and-socket joint with a shallow socket, allowing extensive range of motion. However, this anatomical configuration makes it relatively unstable and prone to dislocation, especially during forceful or awkward movements.