Which of the following is NOT a JAK inhibitor?

• A. Baricitinib
• B. Tofacitinib
• C. Upadacitinib
• D. Methotrexate

Answer: (D)

Distinguishing JAK inhibitors from non-JAK inhibitors is what this item is testing. JAK inhibitors block the JAK-STAT signaling pathway, reducing cytokine-driven inflammation. The drugs Baricitinib, Tofacitinib, and Upadacitinib all inhibit JAK enzymes (with varying selectivity), dampening downstream STAT activation to treat inflammatory conditions like rheumatoid arthritis. Methotrexate, by contrast, is a traditional DMARD that works mainly by inhibiting dihydrofolate reductase and modulating purine metabolism, with anti-inflammatory effects largely through mechanisms outside JAK-STAT signaling (including increased adenosine). Since it does not inhibit JAK enzymes, Methotrexate is not a JAK inhibitor.