What is the cause of persistent diarrhea in a 59-year-old male patient with a history of diverticulosis, internal hemorrhoids, and a recent colonoscopy showing a removed 6 mm polyp, who has not responded to loperamide (Loperamide) or probiotics, but showed temporary improvement with Lomotil (Diphenoxylate and Atropine) 5 mg?

Differential Diagnosis for Chronic Diarrhea

The patient's symptoms and test results suggest several possible diagnoses. Here's a categorized list of potential causes:

• Single most likely diagnosis
  • Irritable Bowel Syndrome (IBS): The patient's symptoms of chronic diarrhea, response to Lomotil (an antidiarrheal medication), and lack of response to loperamide and probiotics suggest IBS. The colonoscopy results were largely normal, which also supports this diagnosis.
• Other Likely diagnoses
  • Inflammatory Bowel Disease (IBD): Although the colonoscopy results were normal, IBD (e.g., Crohn's disease or ulcerative colitis) can cause chronic diarrhea and may not always be visible on a colonoscopy.
  • Small Intestinal Bacterial Overgrowth (SIBO): The patient's symptoms and lack of response to standard treatments suggest SIBO, which can cause chronic diarrhea.
  • Microscopic Colitis: This condition can cause chronic diarrhea and may not be visible on a standard colonoscopy.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Colorectal Cancer: Although the colonoscopy results were normal, it's essential to consider colorectal cancer, especially given the patient's age and symptoms.
  • Neuroendocrine Tumors: These rare tumors can cause chronic diarrhea and may not be visible on standard imaging tests.
  • Lymphoma: Gastrointestinal lymphoma can cause chronic diarrhea and may not be visible on standard imaging tests.
• Rare diagnoses
  • Whipple's Disease: A rare bacterial infection that can cause chronic diarrhea, weight loss, and malabsorption.
  • VIPoma: A rare pancreatic tumor that can cause chronic diarrhea, hypokalemia, and achlorhydria.
  • Thyrotoxicosis: Hyperthyroidism can cause chronic diarrhea, weight loss, and other symptoms.

It's essential to note that the patient's upcoming upper endoscopy may help rule out paraesophageal neoplasm and other conditions that could be contributing to their symptoms. Further testing and evaluation may be necessary to determine the underlying cause of the patient's chronic diarrhea.