A 22-year-old hostel resident presents with foul-smelling, greasy diarrhoea, bloating, and flatulence for two weeks. Stool microscopy shows pear-shaped trophozoites with two nuclei and four pairs of flagella exhibiting a characteristic 'falling leaf' motility. The single best confirmatory test when microscopy is negative is:

• A Acid-fast staining of stool for oocysts
• B String test (Entero-test) for duodenal aspirate
• C Serum anti-Giardia IgM antibody titre
• D Stool antigen detection by ELISA ✓

Explanation

Giardia lamblia trophozoites show the classic pear shape, binucleate 'owl eye' appearance and falling leaf motility. Because intermittent shedding makes microscopy insensitive, stool antigen detection by ELISA or immunochromatography is the recommended confirmatory test, with sensitivity above 95 percent. The string test is reserved for difficult cases, not routine confirmation. Serum antibodies do not distinguish past from current infection, and acid-fast staining detects coccidian oocysts such as Cryptosporidium, not Giardia.

Reference: Paniker's Textbook of Medical Parasitology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.