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
Prevalence of vitamin-A deficiency in a community is assessed as the following: 
Select and explain. 
A. Night blindness-10%, B. Corneal ulcer-0.1%, C. Bitot spots-0.5%, D. Decreased serum retinol level-0.05%
C: Bitot spots-0.5%
Answer: C: Bitot spots-0.5% 
Explanation: Prevalence criteria for determining the xerophthalmia problem: Criteria Prevalence in population at risk (6 months to 6 years) Nightblindness More than 1% Bitot's spots More than 0.5% Corneal xerosis/corneal ulceration/ keratomalacia More than 0.01% Corneal ulcer More than 0.05% Serum retinol (less than 10 mcg/dl) More than 5% Ref: Textbook of Preventive and Social Medicine by K Park, 19th edition, Page 486.