A newborn is brought to the neonatal ward for routine examination. The clinical finding shown at **B** (smooth perineum with absent anal dimple) prompts immediate investigation. The infant has not passed meconium in the first 24 hours and shows abdominal distension. An invertogram is performed at 18 hours of life. Which of the following is the MOST important immediate next step in management if the gas-perineum distance is found to be > 1 cm?