Entamoeba histolytica is a protozoan parasite that causes intestinal inflammation. A few cases have been misdiagnosed as UC with poor outcomes occurring due to the use of corticosteroids.The most common disease that mimics the symptoms of ulcerative colitis is Crohns disease, as both are inflammatory bowel diseases that can affect the colon with similar symptoms. It is important to differentiate these diseases since their courses and treatments may differ. In some cases, however, it may not be possible to tell the difference, in which case the disease is classified as indeterminate colitis. Crohns disease can be distinguished from ulcerative colitis in several ways. Characteristics that indicate Crohns include evidence of disease around the anus (perianal disease). This includes anal fissures and abscesses as well as fistulas, which are abnormal connections between various bodily structures.Infectious colitis is another condition that may present in similar manner to ulcerative colitis. Endoscopic findings are also oftentimes similar. One can discern whether a patient has infectious colitis by employing tissue cultures and stool studies. Biopsy of the colon is another beneficial test but is more invasive. Other forms of colitis that may present similarly include radiation and diversion colitis. Radiation colitis occurs after irradiation and often affects the rectum or sigmoid colon, similar to ulcerative colitis. Upon histology radiation colitis may indicate eosinophilic infiltrates, abnormal epithelial cells, or fibrosis. Diversion colitis, on the other hand, occurs after portions of bowel loops have been removed. Histology in this condition often shows increased growth of lymphoid tissue.
The most common type of skin manifestation, erythema nodosum, presents in up to 3% of UC patients and almost 8% in patients afflicted with Crohns disease. It develops as raised, tender red nodules usually appearing on the outer areas of the arms or legs, especially in the anterior tibial area (shins). The nodules have diameters that measure approximately 1–5 cm. Erythema nodosum is due to inflammation of the underlying subcutaneous tissue (panniculitis), and biopsy will display focal panniculitis (although is often unnecessary in diagnosis). In contrast to joint-related manifestations, erythema nodosum often occurs alongside intestinal disease. Thus, treatment of UC can often lead to resolution of skin nodules.A lesser common skin complication associated with UC, pyoderma gangrenosum, is typically more severe. One study discovered this skin manifestation in only 0.75% pf patients. Pyoderma gangrenosum is characterized by painful lesions or nodules that become ulcers which progressively grow. It often starts as many small raised red spots (papules) or pus-filled lesions (pustules) that are usually detected on lower extremities. The inner layer of the skin, or dermis, slowly undergoes necrosis (process of dying) causing the ulcerations that are typical of pyoderma gangrenosum. The ulcers are often filled with sterile pus-like material, so biopsy often displays an abscess that is sterile. Whereas erythema nodosum tends to correlate with the activity of the ulcerative colitis and often improves with treatment of the colonic inflammation, pyoderma gangrenosum may occur independently of UC disease activity. In some cases, pyoderma gangrenosum may require injection with corticosteroids.