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
Follow–up of cataract operations in national blindness control program is done by – 
Select and explain. 
A. Active surveillance, B. Passive surveillance, C. Sentinel surveillance, D. Routine check-up
C: Sentinel surveillance
Answer: C: Sentinel surveillance 
Explanation: 25 sentinel surveillance units have been established in departments of ophthalmology and PSM in medical collages in India for assessment of -
Beneficiary profiles.
Visual outcomes based on cataract surgery records.
Follow-up of operated cases.  
Ocular morbity data.