<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 25-year-old woman first presented to your clinic due to morning stiffness, symmetrical arthralgia in her wrist joints, and fatigue. She had a blood pressure of 13274 mm Hg and heart rate of 84min. Physical examination revealed tenderness to palpation of both wrists but full range of motion. Anti-citrullinated protein antibodies were positive and ESR was above normal ranges. She was started on methotrexate therapy. She returns for follow up 2 months later and is found to have megaloblastic anemia. What is the mechanism of action of methotrexate?? A: Elevates tetrahydrofolate levels, B: Elevates methylmalonic acid levels, C: Intercalates into strands of DNA, D: Inhibits vitamin B12 activation, E: Inhibits dihydrofolate reductase,  <response>: This is a clinical vignette describing a 25-year-old woman with symptoms and lab findings consistent with rheumatoid arthritis, who was started on methotrexate therapy and developed megaloblastic anemia after 2 months. The question asks about the mechanism of action of methotrexate that leads to megaloblastic anemia. Choice A is incorrect because methotrexate decreases tetrahydrofolate levels by inhibiting dihydrofolate reductase. Choice B is incorrect because methotrexate does not affect methylmalonic acid levels. Choice C is incorrect because methotrexate does not intercalate into DNA strands. Choice D is incorrect because methotrexate does not inhibit vitamin B12 activation.Choice E is correct because methotrexate inhibits dihydrofolate reductase, an enzyme involved in folate metabolism. Inhibition of this enzyme decreases tetrahydrofolate levels, which are required for DNA synthesis. This leads to ineffective erythropoiesis and megaloblastic anemia.E: Inhibits dihydrofolate reductase 