**Question:** A methotrexate intolerant psoriasis patient has been prescribed an alternate disease modifying drug (DMARD) that resulted in liver dysfunction combined with skin rashes and bone marrow suppression of RBC's & platelets. The most probable drug is

A. Hydroxychloroquine
B. Sulfasalazine
C. Leflunomide
D. Azathioprine

**Correct Answer:** D. Azathioprine

**Core Concept:**

Methotrexate is a disease-modifying antirheumatic drug (DMARD) commonly used in the treatment of psoriasis, rheumatoid arthritis, and other autoimmune disorders. However, due to its potential side effects, patients may be switched to alternative DMARDs. Azathioprine, sulfasalazine, and hydroxychloroquine are alternatives to methotrexate, but it is particularly important to consider the side effects and contraindications of each drug.

**Why the Correct Answer is Right:**

Azathioprine (6-mercaptopurine) is an alternative DMARD used in the treatment of psoriasis, rheumatoid arthritis, and other autoimmune disorders. It is a prodrug that is converted to 6-mercaptopurine monophosphate and then to 6-thioguanine nucleotides. These compounds inhibit DNA synthesis, leading to a reduction in the proliferation of autoreactive T cells and B cells, which helps to suppress autoimmune inflammation.

**Why Each Wrong Option is Incorrect:**

**A. Hydroxychloroquine:** Although hydroxychloroquine is an alternative DMARD, it has a significantly lower risk of liver toxicity compared to azathioprine. However, it cannot explain the side effects seen in this scenario.

**B. Sulfasalazine:** Sulfasalazine is another alternative DMARD but it is more commonly associated with gastrointestinal side effects rather than liver toxicity and skin rashes.

**C. Leflunomide:** Leflunomide (Arava) is another DMARD used in psoriasis, rheumatoid arthritis, and other autoimmune disorders. However, it is not the drug causing liver dysfunction, skin rashes, and bone marrow suppression in this scenario. The side effects observed here are more consistent with azathioprine.

**Clinical Pearl:** It is crucial to consider the side effect profile of each alternative DMARD before prescribing them to patients, as they may have different risks and benefits compared to methotrexate. Always tailor the treatment to the individual patient's risk-benefit profile.

**Explanation of Side Effects:**

Azathioprine is known to cause liver toxicity, which can present with elevated liver enzymes and skin rashes, similar to the observed side effects in this scenario. Furthermore, azathioprine has the potential to suppress bone marrow function, leading to bone marrow suppression and decreased platelet production, causing low platelet count and anemia.

**Why the Correct Answer is Right:**

Azathioprine, an immunosuppressive drug, is a commonly used alternative to methotrexate in psoriasis