A patient presents with perianal itching. A peripheral swab was taken, and the image below shows the findings. What is the most likely diagnosis?

Correct Answer: B. Enterobius vermicularis

Enterobius vermicularis (pinworm) is the most common cause of perianal itching, especially in children and in Indian households with poor sanitation. The diagnosis is confirmed by the scotch tape test (cellophane tape swab of the perianal region in early morning), which reveals characteristic embryonated eggs — oval, flattened on one side, measuring 50–60 μm × 20–30 μm. The eggs are immediately infectious (embryonated within 4–6 hours) and cause intense itching due to female worms migrating to the perianal skin at night to deposit eggs. This nocturnal migration is pathognomonic and explains the classic presentation of perianal pruritus worse at night. The peripheral swab (scotch tape preparation) is the gold standard diagnostic method, showing these distinctive eggs. Treatment in India follows IAP guidelines: albendazole 400 mg single dose, repeated after 2 weeks, with simultaneous treatment of all household contacts to prevent reinfection.

Why the other options are wrong

A. H Nana — Hymenolepis nana (dwarf tapeworm) causes abdominal symptoms and diarrhea, not perianal itching. Diagnosis is by stool examination showing characteristic eggs with polar filaments, not perianal swab. H. nana is rare in India compared to E. vermicularis. The scotch tape test is specific for pinworm eggs and would not show tapeworm morphology. C. Ancylostoma — Ancylostoma duodenale causes hookworm disease with anemia, protein malnutrition, and ground itch (larval penetration), not perianal itching. Diagnosis requires stool examination showing hookworm eggs (60–80 μm, oval with thin shell), not perianal swab. Ancylostoma does not migrate to perianal skin and causes systemic symptoms, not localized pruritus. D. Necator Americanus — Necator americanus (American hookworm) is rare in India and causes similar hookworm disease with anemia and malabsorption, not perianal itching. Diagnosis is by stool examination for hookworm eggs, not perianal swab. The clinical presentation is systemic (iron deficiency anemia, protein loss) rather than localized nocturnal perianal pruritus characteristic of pinworm.

High-Yield Facts

Mnemonics

TAPE for Pinworm Diagnosis Tape swab (scotch tape test) → Anal itching (perianal pruritus) → Peripheral eggs (embryonated) → Enterobius vermicularis. Use this when you see 'perianal itching + swab' in a question. NIGHT Itch = Pinworm Nocturnal migration → Intense itching → Gravid females → Household transmission → Tape test confirms. Helps recall the classic nocturnal presentation.

NBE Trap

NBE pairs "perianal itching" with hookworm (Ancylostoma/Necator) to trap students who confuse ground itch (larval penetration) with pinworm-induced perianal pruritus. The key discriminator is the scotch tape swab showing embryonated eggs — hookworms are diagnosed by stool examination, not perianal swab.

Clinical Pearl

In Indian pediatric practice, pinworm is the most common helminthic infection in urban and rural children. A child presenting with nocturnal perianal itching and sleep disturbance should prompt immediate scotch tape testing — early diagnosis and family-wide treatment prevents recurrent infections and school transmission. Many Indian mothers report "worms in stool" after albendazole treatment, which is actually dead worms being expelled.

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 46 (Helminths); Park's Textbook of Preventive and Social Medicine Ch. 7 (Parasitic Infections)_