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
Among case detection studies, cluster testing is useful in detecting cases of: 
Select and explain. 
A. STD, B. Cancer, C. Diabetes, D. Measles
A: STD
Answer: A: STD 
Explanation: Cluster testing is used for detecting STDs. Here the patients are asked to name other persons of either sex who move in the same socio-sexual environment. These persons are then screened (e.g., blood testing). This testing has been shown almost to double the number of cases found. Ref: Textbook of Preventive and Social Medicine by K Park, 19th edition, Page 281.