Main use of Oral Pyrimethamine clinically?

• A. Ocular Histoplasmosis
• B. Chlamydial Conjunctivitis
• C. CSME
• D. Ocular Toxoplasmosis

Answer: (D)

Pyrimethamine is an antiparasitic that blocks a key enzyme in the parasite’s folate synthesis, which hampers DNA replication. In practice, it’s used with sulfadiazine (to enhance the effect on the parasite) and leucovorin (to protect the patient’s own cells from folate depletion). This combination is a classic, effective treatment for toxoplasma gondii infections, especially when the eye is involved. Ocular toxoplasmosis is the scenario where oral pyrimethamine-based therapy is most commonly applied, aiming to control active retinochoroiditis and associated inflammation.

The other conditions listed do not involve Toxoplasma gondii and are treated with different approaches: ocular histoplasmosis is a fungal/Histoplasma-associated condition treated with antifungals rather than antiparasitics; chlamydial conjunctivitis is bacterial and treated with antibiotics targeting Chlamydia; clinically significant macular edema is a diabetic or inflammatory complication managed with anti-VEGF, steroids, or laser therapy rather than antiparasitic drugs.