Answer: Taenia spp. adult tapeworm
Shown here is a long segment of proglottids which had been shed intact in the patient's feces. To confirm the diagnosis, fluid was expressed from the proglottids, and provided the following eggs. Note the thick striated outer wall and inner hooklets, with is classic for Taenia eggs. As a note - If you see hooklets inside a helminth egg, then you know you are dealing with a cestode.
The presence of diagnostic eggs allows for identification to the genus level. However, speciation requires examination of the uterine branches within a mature gravid proglottid (e.g. through india ink injection), or examination of the scolex (if found). In this case, the specimen was submitted in formalin which interfered with our ability to inject india ink into the uterine branches. Since no scolex was found, we were not able to provide definitive speciation in this case.
Salbrent mentions the following important point: "all Taenia eggs and proglottids must be handled with extreme care b/c T solium eggs are infective to humans." This is essential to remember when handling proglottids! Accidental ingestion of T. solium eggs can lead to cysticercosis, where humans serve as the intermediate host of the tapeworm - a much more deadly disease than intestinal infection with the adult tapeworm. Eggs can remain infectious for months, even in fixatives such as ethanol or formalin. So wear gloves whenever handling tapeworm proglottids, and wash hands well afterward.