Which muscle's dysfunction or damage can cause ptosis?

The superior tarsal muscle, also known as the muscle of Müller, plays a crucial role in maintaining the position of the upper eyelid. It is a smooth muscle that responds to sympathetic innervation and helps to elevate the eyelid in conjunction with the levator palpebrae superioris, which is a skeletal muscle. When there is dysfunction or damage to the superior tarsal muscle, the upper eyelid may droop, leading to a condition known as ptosis.

This muscle's primary function is to provide additional support to the eyelid, particularly in situations requiring more elevation, such as when a person is awake or actively engaging in tasks. If the sympathetic nerve supply to this muscle is compromised, it can result in a significant drop in the eyelid, contributing to the appearance of ptosis.

The other options involve muscles or factors that do not directly cause ptosis. The inferior rectus muscle primarily affects eye movement rather than eyelid position, while the muscle of Horner is associated with Horner's syndrome but does not directly raise the eyelid itself. Lastly, Riolan's muscle is involved in eyelid function but is not the primary muscle responsible for elevating the eyelid, thus not directly leading to ptosis when