Choice A reason: IV supplies prepare for hydration or surgery, but assessing the obstruction’s extent via abdominal measurement is the first step to guide subsequent interventions. Choice B reason: Anorectal manometry diagnoses anal function, but bilious vomiting suggests a higher obstruction (e.g., Hirschsprung’s); it’s not the initial assessment needed here. Choice C reason: Measuring abdominal circumference assesses distension from potential obstruction (e.g., meconium ileus), the first action to evaluate severity and guide urgent care. Choice D reason: Monitoring urinary output tracks hydration but doesn’t address the immediate gastrointestinal issue; abdominal assessment prioritizes identifying the obstruction’s impact first.