Serologic testing is the primary method of diagnosing infection with which organism?

• A. Schistosoma japonicum
• B. Cyclospora cayetanensis
• C. Naegleria fowleri
• D. Toxoplasma gondii

Answer: (D)

Serologic testing is most useful for detecting exposure to Toxoplasma gondii by measuring antibodies the immune system makes against the parasite. This organism often causes latent or asymptomatic infections, and direct visualization or culture from a patient sample is difficult or unreliable. Serology provides a noninvasive, widely available way to establish whether someone has been infected, to distinguish recent from past infection (using IgM, IgG, and IgG avidity tests), and to monitor congenital infection risk.

In contrast, the other organisms are typically diagnosed by direct detection methods: looking for parasite eggs in stools or urine (Schistosoma japonicum), identifying oocysts or cysts in stool with special stains (Cyclospora cayetanensis), or examining cerebrospinal fluid with culture, microscopy, or PCR for a rapid, life-threatening CNS infection (Naegleria fowleri). Hence serology isn’t the primary diagnostic tool for those organisms.