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: Pediatrics
In which stage, corneal ulcer involving 1/2 of Corneal surface seen in WHO classification of vitamin A deficiency 
Select and explain. 
A. Stage 2, B. Stage 3A, C. Stage 3B, D. Stage 4
C: Stage 3B
Answer: C: Stage 3B 
Explanation: WHO classification of Vitamin A deficiency:
I Primary changes
X1A ⇒ Conjunctival xerosis
X1B ⇒ Bitot's spots
X2 ⇒ Corneal xerosis
X3A ⇒ Corneal ulcer < 1/3 rd of corneal surface
X3B ⇒ Corneal ulcer > 1/3rd of corneal surface.
II Secondary changes
XN ⇒ Night blindness
XF ⇒ Xeropthalmic fundus
XS ⇒ Xerophthalmia - related corneal Scar