In a patient presenting with persistent bloody diarrhea, rectal urgency, or tenesmus, stool examinations and sigmoidoscopy and biopsy should be performed to confirm the presence of a colitis and to rule out infectious causes. Characteristic endoscopic and histologic findings with negative evaluation for infectious causes will suggest the diagnosis of ulcerative colitis.
The diagnosis of UC is suspected on clinical grounds and supported by the appropriate findings on proctosigmoidoscopy or colonoscopy, biopsy, and by negative stool examination for infectious causes. Infectious etiologies of colitis can produce clinical findings indistinguishable from idiopathic UC, so microbiologic studies for bacterial and parasitic infection, as well as serologic testing for ameoba when clinical suspicion is high, should be performed in each new patient and in patients with stable symptoms who develop a severe exacerbation. Similarly, patients who have had recent antibiotics, or have recently been hospitalized, should have stools examined for Clostridium difficile.
Proctosigmoidoscopy or colonoscopy will reveal the mucosal changes characteristic of ulcerative colitis, consisting of loss of the typical vascular pattern, granularity, friability, and ulceration  . These changes typically involve the distal rectum and proceed proximally in a symmetric, continuous, and circumferential pattern to involve all or part of the colon. Because these endoscopic findings are not at all specific for UC, histologic findings obtained with biopsies may be helpful in the differential diagnosis.
In the patient with acute onset of bloody diarrhea, the mucosal biopsy may help distinguish ulcerative colitis from infectious colitis. In the former, more than in the latter, the mucosa demonstrates separation, distortion, and atrophy of crypts; acute and chronic inflammatory cells in the lamina propria; preferential homing of neutrophils to the crypt epithelium; increased number of plasma cells near the crypt bases; and basilar lymphoid aggregates   . Crypt abscesses, on the other hand, are a nonspecific indication of inflammation and do not indicate a specific diagnosis  . Crohn’s disease may be suggested by certain histologic findings such as non-caseating granulomas or microscopic focality, but their absence does not rule out the possibility of Crohn’s disease. Other histologic findings, which may suggest an infectious etiology, include granulomas in tuberculosis (and even less commonly in schistosomiasis, syphilis, and Chlamydia trachomatous), amoebic trophozoites, pseudomembranes in C. difficile colitis, and viral inclusions in cytomegalovirus or herpetic colitis. Generic Paxil PAROXETINE
In the appropriate clinical settings, sigmoidoscopy or colonoscopy and biopsy may also distinguish the various noninfectious colitides from UC. These include ischemia, radiation, collagenous and microscopic colitis, drug-induced colitis, and the solitary rectal ulcer syndrome  .