Ascariasis — DTM&H MCQ

Correct answer: C — Mebendazole 500 mg single dose

The correct answer is C, Mebendazole 500 mg single dose. The passage of a large adult worm up to 25 to 35 cm and stool eggs that are large, oval, and mammillated (bile-stained, thick outer coat) are classic for Ascaris lumbricoides. Mebendazole 500 mg as a single oral dose is the standard first-line benzimidazole treatment for ascariasis, working by irreversibly blocking glucose uptake in the nematode, causing paralysis and expulsion of the worm. This regimen achieves cure rates well above 90 percent and is the practical, single-dose option favoured in both UK travel/tropical medicine practice and WHO deworming programmes. Albendazole 400 mg single dose is an accepted alternative, but mebendazole is the option offered here and is equally appropriate for this presentation. Why the other options are wrong: E. Praziquantel 40 mg/kg: this is active against trematodes and cestodes (schistosomes, flukes, tapeworms) via calcium channel disruption in the worm tegument, and has no useful activity against nematodes such as Ascaris. D. Pyrantel pamoate 10 mg/kg: this depolarising neuromuscular blocker is effective for Ascaris and Enterobius but is not first-line in UK practice and is not the preferred single-dose benzimidazole answer when mebendazole is available. B. Ivermectin 200 microgram/kg: this is the drug of choice for strongyloidiasis and onchocerciasis, acting on glutamate-gated chloride channels, but is not the standard treatment for ascariasis. A. Nitazoxanide 200 mg twice daily for 3 days: this broad-spectrum antiprotozoal/antihelminthic is used mainly for Giardia and Cryptosporidium and cestode infections, not as first-line therapy for Ascaris. Key point: large mammillated eggs plus a giant roundworm equal Ascaris lumbricoides, treated with a single 500 mg dose of mebendazole.

Reference: BNF: Mebendazole, Antihelminthic drugs, Threadworm and other nematode infections (Ascaris lumbricoides) - single dose 500 mg; NICE British National Formulary, bnf.nice.org.uk/drugs/mebendazole/