This patient has moderate-to-severe Crohn disease that is refractory to 5-aminosalicylic acid (5-ASA) therapy (mesalamine). For such patients, the next step in management is to escalate therapy. Biologic agents, such as anti-TNF therapy (e.g., infliximab, adalimumab), are highly effective for inducing and maintaining remission in moderate-to-severe Crohn disease. Increasing the mesalamine dose or switching to another 5-ASA like sulfasalazine is unlikely to be effective. Antibiotics are used for infectious complications like abscesses or for some cases of perianal disease, not for uncomplicated luminal inflammation.

Dermatitis herpetiformis (DH) is a pathognomonic extraintestinal manifestation of celiac disease. It is caused by the deposition of IgA antibodies in the dermal papillae, which triggers an inflammatory response. The vast majority of patients with DH have histologic evidence of celiac disease on small bowel biopsy, even if they do not have overt gastrointestinal symptoms. Management involves both a strict gluten-free diet and medications like dapsone for symptomatic relief of the rash.

The clinical picture of severe colonic dilation (transverse colon > 6 cm) accompanied by signs of systemic toxicity (fever, leukocytosis) is diagnostic of toxic megacolon. This is a life-threatening complication of severe colitis, most commonly seen in ulcerative colitis. The inflamed, dilated, and thinned bowel wall is at high risk for perforation, which can lead to peritonitis, sepsis, and death. Therefore, perforation is the most immediate and feared complication requiring urgent surgical consultation.

This patient's presentation is highly suspicious for an intra-abdominal abscess, a known complication of penetrating Crohn disease. The first step in management is to confirm the diagnosis and delineate the anatomy of the abscess. A CT scan of the abdomen and pelvis with intravenous and oral contrast is the imaging modality of choice for this purpose. It will confirm the presence, size, and location of the abscess and help guide subsequent therapy, which typically involves percutaneous drainage and broad-spectrum antibiotics. Starting immunosuppressants like prednisone or infliximab is contraindicated in the setting of an undrained abscess.

Patients with long-standing, extensive inflammatory bowel disease (involving more than the rectum) are at increased risk for colorectal cancer. Surveillance colonoscopy is recommended to begin 8-10 years after the initial diagnosis. This patient was diagnosed 8 years ago with pancolitis, so it is appropriate to begin surveillance now. The frequency of subsequent colonoscopies (typically every 1-3 years) depends on prior findings, disease activity, and other risk factors. Fecal testing is not sufficient for surveillance in IBD.

The endoscopic findings described are classic for ulcerative colitis. Key features include: 1) inflammation starting in the rectum (proctitis), 2) continuous inflammation extending proximally without intervening normal mucosa ('skip lesions'), and 3) absence of small bowel involvement. Crohn disease is typically characterized by skip lesions, rectal sparing, deeper ulcers, and frequent terminal ileum involvement. Ischemic and infectious colitis can have varied appearances but do not typically present with this chronic, continuous pattern starting in the rectum.

For simple, asymptomatic or mildly symptomatic perianal fistulas in Crohn disease, a course of antibiotics is often the first-line medical therapy. Metronidazole, with or without ciprofloxacin, can help reduce fistula drainage and inflammation. Biologic agents like infliximab are highly effective but are typically reserved for more complex, symptomatic, or refractory fistulas due to cost and potential side effects. Topical steroids and oral mesalamine are ineffective for treating fistulizing Crohn disease.

For mild-to-moderate distal ulcerative colitis (proctitis or proctosigmoiditis), topical therapy with a 5-aminosalicylic acid (5-ASA) agent is the first-line treatment. Mesalamine enemas can reach the sigmoid colon and are more effective than oral 5-ASA agents for distal disease because they deliver a higher concentration of the active drug to the site of inflammation. Systemic corticosteroids (prednisone), immunomodulators (azathioprine), and biologics (infliximab) are reserved for more extensive or severe disease.

This patient's presentation of new, severe symptoms ('alarm symptoms' like weight loss and pain) after a long period of well-controlled celiac disease is highly concerning for a malignancy. Enteropathy-associated T-cell lymphoma (EATL) is a rare but well-known complication of long-standing celiac disease. The endoscopic findings of ulceration are also suspicious. While other conditions like refractory celiac disease or SIBO are possible, the severity of the presentation makes EATL the most urgent and concerning diagnosis to exclude.

Smoking has a paradoxical and opposing effect on the two main types of IBD. It is a significant risk factor for the development and worsening of Crohn disease. Conversely, smoking appears to be protective against ulcerative colitis. It is a well-recognized clinical observation that a subset of patients develop ulcerative colitis for the first time or experience a significant flare of their disease shortly after quitting smoking. The colonoscopy findings of continuous distal inflammation are also consistent with ulcerative colitis.