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: Social & Preventive Medicine
Serum retinol below which level is considered to be an indicator for xerophthalmia? 
Select and explain. 
A. 1 mcg/ dl, B. 5 mcg/ dl, C. 10 mcg/ dl, D. None of the above
C: 10 mcg/ dl
Answer: C: 10 mcg/ dl 
Explanation: The following are the prevalence criteria for determining xerophthalmia problem: Criteria Prevalence in 6 month to 6 years population Night blindness >1% Bitot's spots >0.5% Corneal xerosis/ulceration >0.01% Corneal ulcer >0.05% Serum retinol <10 mcg/dl >5% Ref: Park 22nd edition, page 571