Differential Diagnosis for Rectal Bleeding in a Patient with Crohn's Disease

Single Most Likely Diagnosis

• Crohn's Disease Flare: Given the patient's history of Crohn's disease and current symptoms of rectal bleeding, an elevated WBC count, and elevated lactic acid, a flare of the disease is highly plausible. The recent colonoscopy showing no active inflammation does not entirely rule out a flare, as inflammation can be patchy and intermittent.

Other Likely Diagnoses

• Colitis: This could be either a complication of Crohn's disease or a separate entity. The presence of rectal bleeding and elevated inflammatory markers supports this diagnosis.
• Imuran-induced Complications: Although less common, long-term use of Imuran (azathioprine) can lead to complications such as myelosuppression or increased risk of infections, which might indirectly contribute to the patient's symptoms.
• Infection: Given the elevated WBC and lactic acid, an infectious process, potentially complicating the Crohn's disease or as a separate entity, should be considered.

Do Not Miss Diagnoses

• Colorectal Cancer: Despite the recent normal colonoscopy, the concern for malignancy cannot be entirely dismissed, especially in a patient with a history of inflammatory bowel disease, which increases the risk of colorectal cancer. The abnormal CR (possibly referring to a complete blood count or another diagnostic test) concerning for malignancy warrants thorough investigation.
• Toxic Megacolon: A life-threatening complication of severe colitis or Crohn's disease flare, characterized by significant dilation of the colon, which can lead to perforation. The elevated lactic acid and WBC count could indicate severe inflammation.
• Severe Infection/Septicemia: The elevated WBC and lactic acid suggest a significant infectious process, which could be related to the Crohn's disease itself or a complication thereof.

Rare Diagnoses

• Drug-induced Colitis: Although rare, certain medications can induce colitis. Given the patient is on Imuran, while rare, drug-induced colitis should be considered.
• Ischemic Colitis: This condition, resulting from insufficient blood flow to the colon, can present with rectal bleeding and abdominal pain. It is less likely but should be considered, especially if other diagnoses are ruled out.
• Other Rare Infections: Such as CMV colitis, which can occur in immunocompromised patients, including those on long-term immunosuppressive therapy like Imuran.