Upon assessment of an infant, the therapist notes unilateral chest rise and fall, suspecting a pneumothorax. What should the therapist do to quickly determine if this is the case?

Transillumination is a rapid bedside assessment tool that can be particularly useful in infants to differentiate between pneumothorax and other conditions like pleural effusion. In infants, especially those who are preterm or have respiratory distress, this technique involves shining a light through the chest wall to observe how the light behaves. If there is a pneumothorax, the light will not penetrate through the air-filled space effectively, which presents a different appearance compared to normal lung tissue.

Immediate assessments such as a chest x-ray, while definitive, take more time and may not be practical in an emergency setting. Checking heart rates provides important information about the infant's overall health but does not specifically indicate respiratory issues. Listening for breath sounds can offer clues about lung function but may not provide a clear and expeditious differentiation between pneumothorax and other conditions. Thus, transillumination is the most appropriate immediate action for assessment in this scenario, enabling the therapist to make a quick clinical decision.