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: Ophthalmology
Xerophthalmia Grade X-3A 
Select and explain. 
A. Corneal xerosis, B. Bitot spot, C. Corneal ulcer involving less than 1/3 of corneal surface, D. Corneal ulcer involving more than 1/3 of corneal surface
C: Corneal ulcer involving less than 1/3 of corneal surface
Answer: C: Corneal ulcer involving less than 1/3 of corneal surface 
Explanation: Ans. is 'c' i.e., Corneal ulcer involving less than 1/3 of corneal surface WHO classification of xerophthalmia XN Night blindness XIA Conjunctival xerosis XIB Bitot's spots X2 Corneal xerosis X3A Corneal ulceration/keratomalacia affecting less than one-third corneal surface X3B Corneal ulceration/keratomalacia affecting more than one-third corneal surface XS Corneal scar due to xerophthalmia XF Xerophthalmic fundus