A 9-year-old male has chest pain that increases with deep breathing and has a cough. What is the most likely cause of his pain?

In this scenario, the presentation of chest pain that worsens with deep breathing, combined with a cough, suggests a non-cardiac origin for the pain. Conditions such as musculoskeletal issues, inflammation of the pleura, or respiratory processes can cause discomfort in the chest area that is pleuritic, meaning it is aggravated by breathing movements.

The type of pain described here is often associated with conditions like pleurisy or muscle strain, both of which would fit the symptom profile presented. In children, non-cardiac chest pain is much more common than cardiac-related pain, which would typically present differently, often with additional symptoms such as radiating pain or associated systemic features.

While pneumonia could be a consideration, the specific indication of pain worsening with deep breathing points more towards a non-cardiac cause, as pneumonia would generally include systemic signs such as fever or more pronounced respiratory difficulty.

Thus, the angle of deep breathing aggravating the pain is a strong indicator that this particular case is related to a non-cardiac etiology, aligning with the chosen answer.