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The scenario described points towards a left-sided pneumothorax as the likely cause of the newborn's deteriorating heart rate and oxygen saturation, especially given the presence of diminished breath sounds and a bright glow on transillumination.

In a left-sided pneumothorax, air accumulates in the pleural space on the left side, which leads to the collapse of the left lung. This collapse can result in diminished breath sounds when listening to that side, as there is less lung tissue available to generate the normal sounds associated with breath. The connected symptoms of deteriorating heart rate and oxygen saturation can be linked to the impaired gas exchange due to the reduced lung volume and resultant hypoxia.

The finding of a bright glow on transillumination is particularly telling; when a pneumothorax is present, transillumination using a light source will show a bright spot over the affected area because air, being a poor conductor of light, allows the emitted light to pass through more effectively compared to the surrounding tissues. This is a classic diagnostic sign in neonates indicating the presence of air in the pleural space.

While pneumonia, congenital heart defects, and respiratory distress syndrome can also cause respiratory distress in newborns, they typically present in different ways. Pne