This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Orbit, Subject name: Ophthalmology
A patient developed proptosis and diplopia of 2 months duration following injury. On examination there is chemosis, conjunctival redness, and external ocular nerve palsy. Investigation of choice is: 
Select and explain. 
A. MRI, B. CECT, C. MR angiography, D. Intra -arterial digital subtraction angiography
D: Intra -arterial digital subtraction angiography
Answer: D: Intra -arterial digital subtraction angiography 
Explanation: Ans. (d) Intra-arterial digital subtraction angiographyRef: Kanski 7/e, p. 95, 96History of trauma with proptosis, chemosis and ophthalmoplegia indicates carticocavernous fistula* Investigations: CT and MR may demonstrate prominence of the superior ophthalmic vein and diffuse enlargement of extraocular muscles. The ultimate test, however, is selective arteriography of both internal and external carotid arteries.