A 7-year-old boy was referred to a tertiary centre for further assessment having been found to be hypoxic (SpO2 83% in air) during GP consultation where he presented with a 3-day history of non-productive cough. He was afebrile. Previously fit and healthy, he played regular football with no reported exertional dyspnoea. There was no significant family history.

On examination he had mild finger clubbing but no evidence of cyanosis or respiratory distress. There were no visible mucocutaneous stigmata. His chest was clear on auscultation. Normal heart sounds were heard with no audible murmurs. Neurological examination was normal. Blood investigations demonstrated a normal capillary blood gas, an elevated haemoglobin (162 g/L) and haematocrit (0.462 l/l), and a low C-reactive protein (<1 mg/L).

A plain film chest radiograph was performed on admission (figure 1) depicting an area of focal left upper zone reticulation (indicated via red arrow). Initially, there was concern that this could represent underlying bronchiectasis, prompting performance of a non-contrast …