A 26 year old IT employee had sudden onset of 'thunderclap headache' and later he became unconsciousness. He didn't have any history of head trauma. On examination, unilateral loss of light reflex and pupillary dilation is noted. The most probable diagnosis in this patient is:

This is a clinical presentation of subarachnoid hemorrhage (SAH). Rupture of an aneurysm at the junction of the posterior communicating aery and the internal carotid aery causes third cranial nerve involvement. Symptoms include: Pupillary dilation Loss of ipsilateral (but retained contralateral) light reflex Focal pain above or behind the eye Also know: Excluding head trauma, the most common cause of SAH is rupture of a saccular aneurysm. The three most common locations for aneurysms are the terminal internal carotid aery, middle cerebral aery (MCA) bifurcation, and top of the basilar aery. Aneurysms can undergo small ruptures and leaks of blood into the subarachnoid space, so-called sentinel bleeds. Ref: Hemphill, III J.C., Smith W.S., Gress D.R. (2012). Chapter 275. Neurologic Critical Care, Including Hypoxic-Ischemic Encephalopathy, and Subarachnoid Hemorrhage. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.