This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitreous and retina, Subject name: Ophthalmology
The average distance of the fovea from the temporal margin of the optic disc is - 
Select and explain. 
A. 1 disc diameter, B. 2 disc diameter, C. 3 disc diameter, D. 4 disc diameter
B: 2 disc diameter
Answer: B: 2 disc diameter 
Explanation: Fovea centralis is the central depressed pa of the macula. It is about 1.5 mm in diameter and is the most sensitive pa of the retina. In its centre is a shining pit called foveola (0.35-mm diameter) which is situated about 2 disc diameters (3 mm) away from the temporal margin of the disc and about 1 mm below the horizontal meridian. Ref:comprehensive ophthalmology AK KHURANA page no :250