Final answer:
The most likely cause of the described symptoms—right eye pain, ptosis, and inability to adduct the eye—is a Posterior communicating artery aneurysm, given the acute onset and absence of other systemic or neurological signs.
Step-by-step explanation:
A 50-year-old man presents to the emergency room with acute symptoms of right eye pain, ptosis, and diplopia, consistent with an inability to adduct the right eye nasally and a ptotic right eyelid. Given the symptoms and the clinical presentation, the most likely cause of this condition is C. Posterior communicating artery aneurysm. This diagnosis is suggested because the symptoms are acute, occurred upon awakening, involve the right eye specifically, and the pupil is normal-sized and reactive. However, the Acute loss of the ability to move the eye nasally (medially) typically indicates a problem affecting the third nerve or associated structures, while a normal pupil suggests against third nerve compression, common in posterior communicating artery aneurysm. Although diabetes, stroke, multiple sclerosis, or myasthenia gravis could potentially cause similar symptoms, they are less likely in this acute unilateral presentation without other systemic or neurological signs. In the case of diabetes or a small stroke, one might expect other symptoms or signs based on a broader vascular distribution or brain involvement, and multiple sclerosis or myasthenia gravis would typically present more gradually or variably, rather than acutely upon awakening.