Oculomotor nerve (III) palsy: This condition often presents with a fixed, dilated pupil on the affected side due to the nerve's control over pupillary constriction. It can be caused by various factors including aneurysms, diabetes, or trauma.

Traumatic mydriasis: Direct trauma to the eye can cause damage to the iris sphincter or the nerves controlling it, leading to a fixed dilated pupil.
Adie's pupil (Tonic pupil): A condition characterized by a dilated pupil that reacts poorly to light but has a more pronounced response to near vision. It often affects young women and can be associated with diminished or absent deep tendon reflexes.
Pharmacologic mydriasis: Accidental or intentional exposure to substances that dilate the pupils, such as certain eye drops or drugs, can cause a fixed dilated pupil.

Pupillary involvement in a posterior communicating artery aneurysm: Although less common, an aneurysm in this location can compress the oculomotor nerve, leading to a fixed, dilated pupil. Missing this diagnosis could be catastrophic due to the risk of aneurysmal rupture.
Orbital apex syndrome: This condition involves multiple cranial nerves and can present with a fixed dilated pupil among other symptoms. It requires prompt diagnosis and treatment to prevent serious complications.

Syphilis (Luetic pupil): A rare cause of a fixed, dilated pupil, associated with tertiary syphilis.
Familial dysautonomia (Riley-Day syndrome): A rare genetic disorder that affects the development of the autonomic nervous system and can present with abnormalities in pupillary function among other symptoms.
Congenital anomalies: Certain congenital conditions can affect the iris or the nerves controlling pupillary size, leading to a fixed dilated pupil.