A fisherman presents with complaints of chronic diarrhea and fatigue. His physician orders a complete blood count with differential, which reveals a megaloblastic anemia. Which of the following organisms is the most likely cause of this patient's problems?

Vitamin B12 deficiency with resulting megaloblastic anemia is specifically associated with infection with the fish tapeworm, Diphyllobothrium latum. The adult worm attaches to the small intestinal mucosa and releases eggs into the feces. Crustaceans living in contaminated water ingest free-swimming embryos that hatch from the eggs. Ceain species of fish ingest the crustaceans, then humans acquire the infection by ingesting the undercooked or raw fish. Vitamin B12 deficiency can develop because the tapeworm absorbs vitamin B12 from the intestinal lumen and also interferes with ileal B12 absorption. Clonorchis sinensis infections produce obstruction of the biliary tract or pancreatic duct and are associated with an increased risk of cholangiocarcinoma. Echinococcus granulosus larvae infect humans and produce unilocular cystic lesions in the liver. Echinococcus is prevalent in areas in which dogs are used to help raise livestock. Taenia saginata is the beef tapeworm ; it causes diarrhea but not vitamin B12 deficiency. Ref: Brooks G.F. (2013). Chapter 46. Medical Parasitology. In G.F. Brooks (Ed),Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.