Differential Diagnosis for 17-Year-Old Patient with Abdominal Pain and Diarrhea

Given the symptoms and test results of the 17-year-old patient, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis

  • Irritable Bowel Syndrome (IBS): This is a common condition characterized by chronic abdominal pain and changes in bowel movements, which can include diarrhea. The absence of inflammatory markers (normal CRP and CBC counts) and negative stool tests for parasites or C. difficile, along with the pattern of symptoms, makes IBS a strong consideration. The elevated ALT could be incidental or related to another condition but does not directly point away from IBS.

• Other Likely Diagnoses

  • Inflammatory Bowel Disease (IBD): Although the CRP is normal and stool tests are negative, early or mild IBD (including Crohn's disease and ulcerative colitis) could still be a possibility. The diagnosis of IBD often requires a combination of clinical suspicion, imaging, and endoscopic evaluation.
  • Functional Dyspepsia: This condition involves upper abdominal symptoms such as pain or discomfort not exclusively related to bowel movements, but it could be considered if the patient's symptoms are not entirely explained by IBS.
  • Lactose Intolerance or Other Food Intolerances: These could cause similar gastrointestinal symptoms, including diarrhea, and are relatively common in the population.

• Do Not Miss Diagnoses

  • Appendicitis: Although less likely given the chronic nature of symptoms and lack of significant nausea or vomiting, appendicitis can present atypically, especially in younger patients. It's crucial to consider this diagnosis due to its potential for serious complications if missed.
  • Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause intermittent abdominal pain and changes in bowel habits. It's more common in younger children but can occur in adolescents.
  • Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, which can cause abdominal pain, diarrhea, and other symptoms.

• Rare Diagnoses

  • Celiac Disease: An autoimmune disorder caused by a reaction to gluten, leading to small intestine damage and symptoms that can include abdominal pain and diarrhea. The elevated ALT could be related to celiac disease, as liver enzyme elevations can occur in this condition.
  • Giardiasis: Although the stool test for parasites was negative, giardiasis can sometimes be missed on initial testing. It's a less common cause of chronic diarrhea in developed countries but should be considered, especially if the patient has been exposed to contaminated water or has traveled to endemic areas.
  • Chronic Intestinal Pseudo-obstruction: A rare condition characterized by impairment of the muscle contractions that move food, fluid, and air through the intestines, leading to symptoms that can mimic mechanical obstruction without any physical blockage.

Each of these diagnoses should be considered in the context of the patient's full clinical picture, including additional history, physical examination findings, and potentially further diagnostic testing.