This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable Diseases, Subject name: Social & Preventive Medicine
A population of 100000 is under surveillance during an year. 100 cases were positive for malarial thick smear. What is the annual parasite index - 
Select and explain. 
A. 1 per 1000, B. 2 per 1000, C. 10 per 1000, D. 20 per 1000
A: 1 per 1000
Answer: A: 1 per 1000 
Explanation: Ans. is 'a' i.e., 1 per 1000 * Annual Parasite Incidence = (confirmed cases during one year/population under surveillancee) x 1000* Population under surveillance = 100000* Confirmed cases =100API =100------100,000x 1000 = 1* So, API is 1 per 1000 population.