Review Question - QID 212780

A 5-year-old girl is brought to the emergency department for abdominal pain and diarrhea. The mom reports that the patient was playing by herself while the mom was decorating the nursery for her incoming baby when the child began to moan in pain. This began about 4 hours ago, and the pain has progressively worsened. Since then, she has had 3 episodes of bloody diarrhea. The mom denies any fever, chills, vomiting, or significant medical history except for a cold 2 days ago. She is up to date on her vaccinations. Her temperature is 99°F (37°C), blood pressure is 92/64 mmHg, pulse is 106/min, respirations are 19/min, and SpO2 is 97% on room air. A physical examination demonstrates a child in acute distress with diffuse abdominal tenderness. Laboratory findings are shown below. An upright abdominal radiograph is obtained and shown in Figure A.

Hemoglobin: 11 g/dL
Hematocrit: 35%
Leukocyte count: 6,800/mm^3 with normal differential
Platelet count: 180,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 97 mEq/L
K+: 4.1 mEq/L
HCO3-: 18 mEq/L
BUN: 8 mg/dL 
Glucose: 135 mg/dL
Creatinine: 1.5 mg/dL
Ca2+:  8.4 mg/dL
AST: 2,980 U/L
ALT: 1,684 U/L

What is the most likely explanation for this patient’s presentation?