When looking down, which condition causes the lower eyelid to bulge abnormally?

The correct choice indicates that keratoconus is responsible for the bulging of the lower eyelid when looking down, which is not the case. Instead, the condition that specifically causes bulging or protrusion of the eyes, including the lower eyelid, especially when looking in certain directions, is thyroid eye disease.

Thyroid eye disease is associated with hyperthyroidism, particularly in cases linked to Graves' disease, where the autoimmune response leads to inflammation and swelling of the eye muscles and surrounding tissues. This can give the appearance of bulging eyes, referred to as exophthalmos. When a person with this condition looks down, the extraocular muscles can exert more pressure, enhancing the visible bulging of the lower eyelid due to the related soft tissue changes.

Keratoconus, on the other hand, is a corneal condition characterized by the thinning and conical shape of the cornea, which primarily affects vision rather than the appearance of the eyelids. It does not lead to eyelid bulging, making it an inappropriate choice for this question.