Which parasite is commonly diagnosed by observing rhabditiform larvae in stool and is capable of autoinfection?

• A. Enterobius vermicularis
• B. Ascaris lumbricoides
• C. Trichuris trichiura
• D. Strongyloides stercoralis

Answer: (D)

Strongyloides stercoralis is diagnosed when rhabditiform larvae are seen in stool because part of its life cycle can stay inside the human host and continually restart without re-exposure. The key feature is autoinfection: some rhabditiform larvae can molt into the infective filariform form within the host and penetrate the intestinal mucosa or perianal skin, re-entering the body to sustain infection. This ability to cycle inside the same person allows chronic infections that can persist for years and suddenly worsen if the immune system is suppressed. In laboratory stool exams, locating rhabditiform larvae is a classic clue for this parasite, whereas the other options are typically diagnosed by eggs in stool or by perianal sampling rather than by rhabditiform larvae.