Differential Diagnosis for a 7-year-old boy with 1 day of fever and abdominal pain

Single most likely diagnosis

• Viral Gastroenteritis: This is the most common cause of acute abdominal pain and fever in children, often presenting with generalized abdominal pain, fever, and sometimes vomiting or diarrhea. The low lymphocyte count could be a reactive lymphopenia, which can occur in the early stages of a viral infection.

Other Likely diagnoses

• Appendicitis: Although the abdominal pain is generalized, early appendicitis can present with non-specific symptoms. The low lymphocyte count is not typical, but appendicitis is a common cause of abdominal pain in children and should be considered.
• Mesenteric Lymphadenitis: This condition, often caused by viral or bacterial infections, can mimic appendicitis and presents with abdominal pain and fever. The low lymphocyte count might not be directly related, but it's a plausible diagnosis given the symptoms.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed)

• Intussusception: A life-threatening condition where a part of the intestine telescopes into another, causing bowel obstruction. It's a rare cause of abdominal pain but requires immediate attention. Fever and abdominal pain are common presentations.
• Pneumonia with abdominal symptoms: Sometimes, pneumonia can present with abdominal pain, especially if the pneumonia is in the lower lobes. The low lymphocyte count could be seen in bacterial pneumonia.
• Sepsis: Although the labs are otherwise normal, sepsis can present with non-specific symptoms, including fever and abdominal pain. The low lymphocyte count could be an early sign of sepsis.

Rare diagnoses

• Hematologic malignancies (e.g., Leukemia): These can present with non-specific symptoms, including fever and abdominal pain, due to infiltration of the abdomen or due to complications like sepsis. The low lymphocyte count could be a clue, although it's more commonly associated with an elevated white blood cell count.
• Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with abdominal pain and fever. However, they typically have a more chronic course and specific findings on lab tests and imaging.
• Systemic Juvenile Idiopathic Arthritis (sJIA): This condition can present with fever, abdominal pain, and a variety of other systemic symptoms. The low lymphocyte count might not be directly related, but sJIA is a diagnosis to consider in a child with systemic symptoms.