Differential Diagnosis

• Single most likely diagnosis
  • Inflammatory bowel disease (IBD): The patient's symptoms of increased bowel movements, stools covered with mucus and blood, and the presence of erythematous, tender nodules over the lower extremities (possibly erythema nodosum) are consistent with IBD, such as Crohn's disease or ulcerative colitis. The fact that over-the-counter antidiarrheal drugs have not relieved her symptoms and the presence of extraintestinal manifestations (skin nodules) also support this diagnosis.
• Other Likely diagnoses
  • Viral gastroenteritis: Although less likely given the chronic nature of the symptoms and the presence of blood and mucus in the stool, viral gastroenteritis could still be considered, especially if the patient has been exposed to someone with a similar illness.
  • Amebiasis: This parasitic infection can cause chronic diarrhea with blood and mucus, but it is less common in the USA and typically associated with travel to endemic areas.
• Do Not Miss diagnoses
  • Diverticulitis: Although less likely in a young patient, diverticulitis can cause severe symptoms, including bloody stools and abdominal pain. Missing this diagnosis could lead to serious complications, such as perforation or abscess formation.
  • Hyperperistaltic diarrhea due to a underlying condition (e.g., thyroid storm, pheochromocytoma): These conditions can cause severe diarrhea and are important to consider, as missing them could lead to significant morbidity or mortality.
• Rare diagnoses
  • Other infectious causes (e.g., Clostridioides difficile, Salmonella, Shigella): These infections can cause similar symptoms but are less common in a healthy, pregnant woman without a clear exposure history.
  • Vasculitis (e.g., Henoch-Schönlein purpura): This condition can cause gastrointestinal symptoms, skin lesions, and other systemic symptoms, but it is rare and typically associated with other findings, such as joint pain or renal involvement.