A 50-year-old man presents with complaints of bilateral morning stiffness in his wrists and knees and painful joints on exercise. On physical examination, the joints are slightly swollen. The rest of the examination is unremarkable. His laboratory findings are also negative except for slight anemia, elevated erythrocyte sedimentation rate, and positive rheumatoid factor. X-ray of B/L knee was done. The patient was staed on weekly methotrexate to control the inflammation. What is the principle mechanism of action here?

**Core Concept**
The patient's presentation is consistent with rheumatoid arthritis (RA), a chronic autoimmune disorder characterized by inflammation and degeneration of synovial joints. The underlying principle being tested is the mechanism of action of methotrexate, a disease-modifying antirheumatic drug (DMARD) commonly used to control RA inflammation.

**Why the Correct Answer is Right**
Methotrexate exerts its anti-inflammatory effects by inhibiting dihydrofolate reductase (DHFR), an enzyme crucial for the synthesis of tetrahydrofolate (THF). THF is necessary for the production of purines, which are the building blocks of DNA and RNA. By inhibiting THF synthesis, methotrexate reduces the proliferation of activated T and B lymphocytes, which are involved in the inflammatory response in RA. Additionally, methotrexate may also induce apoptosis in rheumatoid synovial cells and modulate the immune response by increasing the production of anti-inflammatory cytokines.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is incorrect because while methotrexate does have some effects on the gastrointestinal tract, its mechanism of action in RA is not primarily through gastrointestinal toxicity.
* **Option B:** This option is incorrect because methotrexate does not primarily work by inhibiting the enzyme cyclooxygenase (COX), which is involved in the synthesis of prostaglandins and is targeted by nonsteroidal anti-inflammatory drugs (NSAIDs).
* **Option C:** This option is incorrect because while methotrexate may have some effects on the liver, its mechanism of action in RA is not primarily through liver toxicity or fibrosis.
* **Option D:** This option is incorrect because while methotrexate may have some effects on the bone marrow, its mechanism of action in RA is not primarily through bone marrow suppression.

**Clinical Pearl / High-Yield Fact**
Methotrexate is a key component of the treatment of rheumatoid arthritis, and its use has been shown to improve outcomes and reduce disease progression. However, it is essential to monitor patients for potential side effects, including liver toxicity and bone marrow suppression.

**Correct Answer:** C. Inhibition of dihydrofolate reductase (DHFR) enzyme activity.

✓ Correct Answer: B. Increased extracellular levels of adenosine