Differential Diagnosis

The patient's history of psoriasis, long-standing abdominal pain, and elevated fecal calprotectin (> 1000) with a normal colonoscopy and negative CRP, suggests an inflammatory process in the gastrointestinal tract. Here's a categorized differential diagnosis:

• Single most likely diagnosis

  • Crohn's Disease: Given the patient's history of psoriasis (which is associated with an increased risk of inflammatory bowel disease), chronic abdominal pain, and significantly elevated fecal calprotectin, Crohn's disease is a strong consideration. The normal colonoscopy does not rule out Crohn's, as it can affect areas of the GI tract not visualized by colonoscopy, such as the small intestine.

• Other Likely diagnoses

  • Ulcerative Colitis: Although the colonoscopy was normal, ulcerative colitis could be in remission or limited to the proximal colon, which might not have been fully visualized. The elevated fecal calprotectin suggests ongoing inflammation.
  • Microscopic Colitis: This condition can cause chronic diarrhea and abdominal pain with normal colonoscopy findings, as the inflammation is only visible under a microscope. It's less likely given the very high fecal calprotectin, but still a consideration.
  • Irritable Bowel Syndrome (IBS): While IBS can cause abdominal pain, the significantly elevated fecal calprotectin points towards an inflammatory cause rather than a functional disorder like IBS.

• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)

  • Intestinal Lymphoma: Although rare, lymphoma can cause chronic abdominal pain and elevated inflammatory markers. It's crucial to consider, especially if the patient has a history of immunosuppression or other risk factors.
  • Infectious Causes (e.g., TB, Parasites): Chronic infections can cause abdominal pain and elevated inflammatory markers. These would be important to rule out, especially if the patient has traveled to or lives in areas where these infections are common.
  • Ischemic Bowel Disease: This condition can cause chronic abdominal pain and might not always have clear findings on initial investigations. It's a critical diagnosis to consider due to its potential for severe complications.

• Rare diagnoses

  • Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, leading to abdominal pain and gastrointestinal symptoms.
  • Behçet's Disease: A form of vasculitis that can affect the gastrointestinal tract, among other systems, causing a wide range of symptoms including abdominal pain.
  • Gastrointestinal Sarcoidosis: Sarcoidosis can affect any part of the gastrointestinal tract, leading to a variety of symptoms including abdominal pain, although it is rare.

Given the complexity of the case, further investigations such as upper endoscopy, enteroscopy, or imaging studies (e.g., MRI enterography) might be necessary to establish a definitive diagnosis. Additionally, a therapeutic trial or further inflammatory marker testing could help differentiate between the potential diagnoses.