The rectosigmoid portion of the large intestine is the distal (last) part of the sigmoid colon and the proximal (first) part of the rectum. Take a look at a picture on the web of the large intestine and locate those areas - it's the best way to visualize it.
Colitis is a term that denotes an inflammatory state. So your father has got some form of inflammation in that area of his large intestine. The doc should have biopsied the area and the pathologist will hopefully determine what the cause of the inflammation is. The treatment will depend on what they find the cause of the inflammation to be, so you'll have to wait to find out what the pathologist says.
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