Ans. D: Caecum Adult Enterobius vermicularis worms inhabit the caecum, appendix and adjacent poions of the ascending colon, lying closely applied to the mucosal surface. Pinworm (genus En terobius)/Threadworm/Seatworm, It is a nematode (roundworm) and a common human intestinal parasite, especially in children. It inhabits the caecum, appendix and adjacent poion of the ascending colon, lying closely applied to the mucosal surface It causes enterobiasis, or less precisely as oxyuriasis in reference to the family Oxyuridae. The pinworm appears as a white, small and delicate nematode. The adult female has a sharply pointed posterior end, is 8 to 13 millimeters long, and 0.5 millimeter thick. The adult male is considerably smaller, measuring 2 to 5 millimeters long and 0.2 millimeter thick, and has a curved posterior end. The eggs are translucent and have a surface that adheres to environmental objects. The eggs measure 50 to 60 micrometers by 20 to 30 micrometers, and have a thick shell that is flattened on one side. The small size and colorlessness of the eggs make them invisible to the naked eye, except in barely visible clumps of thousands of eggs. Eggs may contain a developing embryo or a fully developed pinworm larva.Inside the host, the larvae grow to 140-150 micrometers in length Pinworms spread through human-to-human transmission, by ingesting (i.e., swallowing) infectious pinworm eggs and/or by anal sex. The eggs are hardy and can remain ble (i.e., infectious) in a moist environment for up to three weeks.They do not tolerate heat well, but can survive in low temperatures: two-thirds of the eggs are still ble after 18 hours at -8 degrees Celsius (18 degF). After the eggs have been initially deposited near the anus, they are readily transmitted to other surfaces through contamination. The surface of the eggs is sticky when laid, and the eggs are readily transmitted from their initial deposit near the anus to fingernails, hands, night-clothing and bed linen.From here, eggs are fuher transmitted to food, water, furniture, toys, bathroom fixtures and other objects.Household pets often carry the eggs in their fur, while not actually being infected. Dust containing eggs can become airborne and widely dispersed when dislodged from surfaces, for instance when shaking out bed clothes and linen.Consequently the eggs can enter the mouth and nose through inhalation, and be swallowed later. Although pinworms do not strictly multiply inside the body of their human host, some of the pinworm larvae may hatch on the anal mucosa, and migrate up the bowel and back into the gastrointestinal tract of the original host. This process is called retroinfection. Despite the limited, 13 week lifespan of individual pinworms, autoinfection (i.e., infection from the original host to itself), either through the anus-to-mouth route or through retroinfection, causes the pinworms to inhabit the same host indefinitely