Which helminth infection is classically associated with B12 deficiency and megaloblastic anemia?

• A. Diphyllobothrium latum
• B. Ascaris lumbricoides
• C. Strongyloides stercoralis
• D. Taenia solium

Answer: (A)

The key idea is that some intestinal parasites can cause nutritional deficiencies by consuming host nutrients. Diphyllobothrium latum, the fish tapeworm, is famous for absorbing large amounts of vitamin B12 in the small intestine. This reduces the host’s B12 available for normal red blood cell formation, leading to megaloblastic (macrocytic) anemia. The diagnostic signal is the combination of B12 deficiency with macrocytosis and characteristic hematologic findings, sometimes accompanied by symptoms of glossitis or neuropathy in advanced cases. The other parasites listed are not classically linked to B12 deficiency: Ascaris lumbricoides is more associated with malnutrition and obstruction, Strongyloides stercoralis with autoinfection and GI symptoms, and Taenia solium with taeniasis and cysticercosis. Thus, the fish tapeworm best explains the B12 deficiency–megaloblastic anemia scenario.