This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Malathion is used once every - 
Select and explain. 
A. 3 months, B. 6 months, C. 9 months, D. 12 months
A: 3 months
Answer: A: 3 months 
Explanation: Malathion has the least toxicity of all organophosphorus compounds. Water disperaible powders are available. Malathion is used in doses of 100-200 mg/sq.ft. every 3 months. (Lindane, malathion OMS-33 duration of effectiveness is 3 months) while DDT has effectiveness for 6 to 12 months. Due to low toxicity malathion is recommended as an alternative to DDT. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 833