Final answer:
The diagnosis for this patient is Pseudomembranous colitis, which is characterized by watery diarrhea, fever, abdominal pain, and loss of appetite. The colonoscopy reveals pigmentation in the wall of the colon.
Step-by-step explanation:
The diagnosis for this patient is Pseudomembranous colitis. Pseudomembranous colitis is characterized by watery diarrhea, fever, abdominal pain, loss of appetite, and in severe cases, perforation of the colon, septicemia, shock, and death. The colonoscopy reveals pigmentation in the wall of the colon, which is a characteristic feature of Pseudomembranous colitis.