Which organism is commonly the etiologic agent of toxoplasmosis?

• A. Pneumocystis jiroveci
• B. Toxoplasma gondii
• C. Acanthamoeba castellanii
• D. Entamoeba histolytica

Answer: (B)

Toxoplasma gondii is the organism that causes toxoplasmosis. It’s a parasitic protozoan that lives inside host cells and cycles between tachyzoite forms during acute infection and bradyzoite-containing tissue cysts in latency. Cats are the definitive hosts, and humans typically acquire infection by eating undercooked meat containing tissue cysts or by ingesting oocysts from cat feces, with potential congenital transmission as well. Most people feel fine or have mild symptoms, but infection can be serious for a fetus or immunocompromised individuals due to reactivation or severe primary disease. The other organisms listed are associated with different infections—Pneumocystis jiroveci with Pneumocystis pneumonia, Acanthamoeba castellanii with keratitis or rare encephalitis, and Entamoeba histolytica with amebic dysentery—so the correct agent for toxoplasmosis is Toxoplasma gondii.