Psychiatryniche
A 24-year-old man with treatment-resistant schizophrenia is on day 18 of clozapine, now 250 mg at night. He is febrile at 38.4 degrees Celsius with a pulse of 118/min, and reports a dull central chest ache and breathlessness walking to the bathroom. His chest is clear and oxygen saturation is 97% on room air. There is no rigidity and he is fully oriented. Today's neutrophil count is normal but the eosinophil count is 0.9 x 10^9/L (reference 0.0 to 0.5). Which one of the following investigations will best establish the cause of his deterioration?