UPSC Prelims 2019— Question 49

This question tests the concept of CRISPR-Cas9. The key principle is that Cas9 is an endonuclease used in the CRISPR genome-editing system. Guided by RNA, Cas9 can cut DNA at a targeted location, enabling targeted genetic modification. The correct answer marked for this question is option A. The distinction matters because UPSC often combines a familiar factual statement with one carefully qualified or overly broad statement. A reliable way to solve such questions is to identify the governing concept first and then test every statement against its precise wording rather than relying on general familiarity. The answer should therefore be selected only after checking whether each component of the option is consistent with the underlying constitutional, economic, scientific, historical or geographical principle. Option A — A molecular scissors used in targeted gene editing: This is the correct choice. It is consistent with the governing fact or with the valid combination identified above. The important point is not merely that the wording appears familiar, but that it matches the specific scope of the question and does not add an unsupported condition. Option B — A biosensor used in the accurate detection of pathogens in patients: This is not the correct choice. It either excludes a statement that must be included, includes a statement that is inaccurate, or identifies a different institution, process, location, technology or historical development from the one tested. Its wording should therefore be rejected when checked against the core principle described above. Option C — A gene that makes plants pest-resistant: This is not the correct choice. It either excludes a statement that must be included, includes a statement that is inaccurate, or identifies a different institution, process, location, technology or historical development from the one tested. Its wording should therefore be rejected when checked against the core principle described above. Option D — A herbicidal substance synthesized in genetically modified crops: This is not the correct choice. It either excludes a statement that must be included, includes a statement that is inaccurate, or identifies a different institution, process, location, technology or historical development from the one tested. Its wording should therefore be rejected when checked against the core principle described above. In exam terms, the safest approach is to break the question into its smallest factual units. Where statements are combined, verify each statement independently and then compare the resulting combination with the four codes. Where the question asks for a single institution, event, technology or location, distinguish the exact term from closely related alternatives. This prevents elimination based only on familiarity and is especially useful in UPSC questions where one small qualifier can change the correctness of an otherwise plausible statement. Thus, after checking the individual propositions and the scope of the alternatives, option A is the answer.