Differential Diagnosis for 48 yo M with Abdominal Pain in RLQ and Liquid Stool

Single Most Likely Diagnosis

• Gastroenteritis: This is the most likely diagnosis given the symptoms of abdominal pain in the right lower quadrant (RLQ) and liquid stool for 3 days. Gastroenteritis, often caused by viral or bacterial infections, leads to inflammation of the gastrointestinal tract, resulting in symptoms like diarrhea, abdominal cramps, and sometimes fever.

Other Likely Diagnoses

• Appendicitis: Although the presence of liquid stool is not typical for appendicitis, it cannot be ruled out entirely, especially if the appendix is in an atypical location. Appendicitis usually presents with pain that initially is diffuse or around the navel and then localizes to the RLQ.
• Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic or acute onset of abdominal pain and changes in bowel habits, including diarrhea.
• Diverticulitis: Inflammation of a diverticulum in the colon can cause abdominal pain, usually on the left side, but can occasionally present on the right, along with changes in bowel habits.

Do Not Miss Diagnoses

• Intestinal Obstruction: Although less likely, an obstruction could cause abdominal pain and changes in bowel habits. It's critical to consider this diagnosis due to its potential for severe complications if not promptly treated.
• Mesenteric Ischemia: This condition, involving insufficient blood flow to the intestines, can present with severe abdominal pain out of proportion to the physical examination findings and can lead to bowel infarction if not recognized and treated promptly.
• Ectopic Pregnancy (in female patients, but mentioned for completeness): Though the patient is male, it's essential to remember this diagnosis in female patients presenting with similar symptoms, as it is a life-threatening condition.

Rare Diagnoses

• Yersiniosis: A bacterial infection caused by Yersinia enterocolitica, which can mimic appendicitis with right lower quadrant pain and diarrhea.
• Meckel's Diverticulitis: Inflammation of a Meckel's diverticulum, a congenital anomaly of the small intestine, can cause abdominal pain and gastrointestinal symptoms.
• Carcinoid Syndrome: Although rare, this condition, caused by a neuroendocrine tumor, can lead to abdominal pain, diarrhea, and other systemic symptoms.

Each of these diagnoses should be considered based on the patient's full clinical presentation, including additional symptoms, physical examination findings, laboratory results, and imaging studies as appropriate.