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
[{'content': 'In which stage, corneal ulcer involving 1/2 of Corneal surface seen in WHO classification of vitamin A deficiency \nSelect and explain. \nA. Stage 2, B. Stage 3A, C. Stage 3B, D. Stage 4', 'role': 'user'}, {'content': "Answer: C \nExplanation: WHO classification of Vitamin A deficiency:\nI Primary changes\nX1A\xa0⇒ Conjunctival xerosis\nX1B\xa0⇒ Bitot's spots\nX2\xa0⇒ Corneal xerosis\nX3A\xa0⇒ Corneal ulcer < 1/3 rd of corneal surface\nX3B\xa0⇒ Corneal ulcer > 1/3rd of corneal surface.\nII Secondary changes\nXN\xa0⇒ Night blindness\nXF\xa0⇒ Xeropthalmic fundus\nXS\xa0⇒ Xerophthalmia - related corneal Scar", 'role': 'assistant'}]