A10-day-old, mechanically ventilated newborn suddenly develops a low heart rate (bradycardia) and low oxygen saturation, despite increasing the oxygen concentration to 100%.what is the first and the most important step in the resuscitation of this newborn

The most important step to take in the case of the scenario given in the question is to assess and establish adequate ventilation. Deprivation of oxygen causes the heart rate to drop because the heart is not getting enough oxygen, and the single most important and effective thing to do in this situation is to ventilate the lungs of the baby.