Differential Diagnosis for a 20-year-old with Right Iliac Fossa Pain and Passage of Bloody Stool

• Single most likely diagnosis:
  • Appendicitis: This is the most likely diagnosis given the location of the pain (right iliac fossa) and the episode of bloody stool. Appendicitis can cause abdominal pain that initially may be diffuse or around the navel and then often shifts to the right lower quadrant. Bloody stool is less common but can occur, especially if there's a perforation or an appendiceal abscess that affects the bowel.
• Other Likely diagnoses:
  • Inflammatory Bowel Disease (IBD) - Crohn's Disease or Ulcerative Colitis: These conditions can cause chronic or recurrent abdominal pain, changes in bowel habits, and bloody stools. The age of the patient and the symptoms could fit a diagnosis of IBD, although the specific location of pain might be more suggestive of Crohn's disease affecting the terminal ileum.
  • Intussusception: Although more common in children, intussusception can occur in adults and cause abdominal pain and bloody stools. It involves the telescoping of one segment of intestine into another, which can lead to bowel obstruction and ischemia.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed):
  • Ectopic Pregnancy: Although less likely in a patient presenting with right iliac fossa pain and bloody stool, ectopic pregnancy is a life-threatening condition that must be considered, especially in any female patient of childbearing age presenting with abdominal pain. The presence of bloody stool would be unusual but not impossible.
  • Diverticulitis: While diverticulitis is more common in older adults, it can occur in younger individuals, especially those with a family history or other risk factors. It typically causes left lower quadrant pain but can cause right-sided pain if the diverticula are located in the right colon.
  • Mesenteric Ischemia: This is a rare but life-threatening condition that can cause severe abdominal pain out of proportion to the physical examination findings. Bloody stools can occur due to bowel ischemia.
• Rare diagnoses:
  • Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause pain, bleeding, or obstruction, typically presenting in younger patients.
  • Gastrointestinal Lymphoma: A type of cancer that can cause a wide range of gastrointestinal symptoms, including abdominal pain and bloody stools, although it is less common in young adults without a history of immunodeficiency or other risk factors.
  • Hemorrhagic Ovarian Cyst: Could potentially cause abdominal pain and, if ruptured, might lead to symptoms that could be confused with those of a gastrointestinal issue, including passage of blood per rectum if there's internal bleeding affecting the bowel.