Differential Diagnosis

Single most likely diagnosis

• Acute Appendicitis: The patient presents with right lower quadrant (RLQ) abdominal pain, direct tenderness in the RLQ, no rebound tenderness, and a history of vomiting. The ultrasound (US) findings of an upper limit normal sized appendix without phlegmon formation, in the context of the clinical presentation, strongly suggest acute appendicitis. The presence of leukocytosis with neutrophilic predominance further supports this diagnosis.

Other Likely diagnoses

• Small Bowel Obstruction: The patient has a history of no bowel movement for 2 days, occasional vomiting, and the US finding of small bowel ileus. These symptoms and findings are consistent with a small bowel obstruction, which could be due to various causes including adhesions, hernias, or other mechanical obstructions.
• Pyelonephritis or Kidney Infection: Although the urinalysis (U/A) does not show a urinary tract infection (UTI), the presence of small bilateral kidneys on US and the patient's symptoms of febrile episodes and abdominal pain could suggest an upper urinary tract infection or pyelonephritis, especially if the infection is not causing significant bacteriuria.

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

• Perforated Viscus: Although the patient does not have rebound tenderness, which is often associated with peritonitis due to a perforated viscus, the presence of severe abdominal pain, fever, and leukocytosis necessitates consideration of this potentially life-threatening condition.
• Mesenteric Ischemia: This condition can present with abdominal pain out of proportion to physical examination findings, and the presence of vomiting, fever, and significant leukocytosis. It is a medical emergency requiring prompt diagnosis and treatment.

Rare diagnoses

• Inflammatory Bowel Disease (IBD) Flare: While less likely given the acute presentation, an flare of IBD (such as Crohn's disease or ulcerative colitis) could present with similar symptoms, including abdominal pain, vomiting, and significant leukocytosis.
• Epidemic Typhus or Other Systemic Infections: In endemic areas, epidemic typhus and other systemic infections could present with fever, abdominal pain, and rash, although the specific constellation of findings and the absence of a rash in this case make these diagnoses less likely.