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You are seeing a 10 year old boy with abdominal pain. He was seen last night with a few hours of periumbilical abdominal pain. Work-up at that time showed a CBC with WBC 10.2, 55% neutrophils, UA negative, RLQ ultrasound appendix not visualized. He was discharged home with return precautions and instructions to follow-up with his pediatrician. He returns 15 hours later with continued constant right mid-abdomen pain, anorexia, nausea, a few episodes of non-bilious vomiting, and temperature of 37.8.

What is the best next diagnostic test to obtain?