Which of the following is a JAK inhibitor?

• A. Methotrexate
• B. Leflunomide
• C. Tofacitinib
• D. Cyclosporine

Answer: (C)

JAK inhibitors block the JAK-STAT signaling pathways that many cytokines use to promote inflammation. Tofacitinib directly inhibits JAK enzymes, reducing signaling for multiple inflammatory cytokines and thereby dampening immune responses.

Methotrexate works as a folate antagonist that reduces immune cell proliferation and activity through effects on DNA synthesis, not through the JAK-STAT pathway. Leflunomide inhibits dihydroorotate dehydrogenase, lowering pyrimidine synthesis and lymphocyte proliferation, again not via JAK enzymes. Cyclosporine is a calcineurin inhibitor that prevents T-cell activation by a different signaling route, not by blocking JAK activity.

Thus, the drug that acts as a JAK inhibitor is tofacitinib.