Following stimulation and suctioning, a full-term infant shows mild respiratory distress and cyanosis. What initial treatment should be started?

In the scenario described, where a full-term infant exhibits mild respiratory distress and cyanosis following stimulation and suctioning, administering oxygen by mask is the initial treatment that addresses the immediate needs of the infant. Oxygen therapy is crucial because it directly addresses the infant's low oxygen saturation, which is indicated by the cyanosis. The provision of supplemental oxygen can improve the infant's oxygen levels rapidly and effectively, helping to alleviate the signs of respiratory distress.

While other interventions like placing the infant on a radiant warmer or providing manual ventilation with 100% oxygen may be important in certain cases, they may not be the first step in this specific situation. The radiant warmer offers thermal stability, which might be necessary later, but it does not directly address the respiratory issue. Similarly, manually ventilating with 100% oxygen is more invasive and typically reserved for more severe respiratory failure where immediate, high levels of support are required. Hyperextending the newborn's neck is not standard practice, as it can obstruct the airway rather than help with breathing.

Therefore, starting with oxygen by mask is a safe and effective approach, ensuring the infant receives the essential respiratory support to improve oxygenation.