text: A common cause of male infertility is the enlargement of a network of these structures, resulting in a "bag of worms" appearance.
full_question: A common cause of male infertility is the enlargement of a network of these structures, resulting in a "bag of worms" appearance. The longest of these structures, the great saphenous one, has an especially thick tunica adventitia. Surgical immobilization and long periods of sitting may predispose a patient to a problem in "deep" types of this structure. Enlarged and twisted examples of these structures are termed (*) "varicose." The largest of these valved, vascular structures is the inferior vena cava, and the pulmonary one is one of the very few that are rich in oxygen. For 10 points, name these vessels that primarily carry deoxygenated blood to the heart, as opposed to arteries.
first_sentence: A common cause of male infertility is the enlargement of a network of these structures, resulting in a "bag of worms" appearance.
raw_answer: veins [prompt on blood vessels]