This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A nondiabetic, non hypeensive adult male develops sudden severe headache with altered sensorium, likely diagnosis is : 
Select and explain. 
A. Brain tumour, B. Sub arachnoid haemorrhage, C. Encephalitis, D. Meningitis
B: Sub arachnoid haemorrhage
Answer: B: Sub arachnoid haemorrhage 
Explanation: Answer is B (Subarachnoid Haemorrhage): Sudden severe headache with altered sensorium indicates haemorrhage as the likely vascular event in this patient. The patient is a non diabetic & non hypeensive, thereby indicating that he has no risk factor to account for the weakening of small perforating vessels, that could lead to their rupture and an intracerebral haemorrhage. The haemorrhage is thus a spontaneous one. Most likely diagnosis therefOre is spontaneous rupture of a saccular aneurysm or 'Subarachnoid H'rrage'.