If any of you have the stomach for it, check out anal atresia or
imperforate anus. It's a congenital malformation that may occur in infants. Babies who are born with this condition basically have a shortened colon and no anus, or an anus that is not connected to the rectum. The usual treatment for this disorder is a permanent colostomy (or sometimes ileostomy) bag, but a surgical solution has been created, which includes a man-made anus.
This is where it gets really islamic. The surgeons can create a "tunnel" between the colon and the perineum, but they cannot construct the musculature that allows the anus to expand and close, or the muscle involved in pushing out feces. And just like a neovagina, this opening needs to be regularly dilated to prevent it from closing up. Yes, the parents of these unfortunate kids need to dilate their child's surgically constructed anus with a dilation rod. Because this is a really traumatizing experience both for the child and the unfortunate adults tasked with it, most parents simply choose to opt for a colostomy.
Anyway, my point being... doctors cannot create one type of tissue out of another or origami it into a functional structure.