A 8-month-old male infant presents with a severe cough, tachypnea, and hypoxemia. A bronchoalveolar lavage fluid PCR is positive for Pneumocystis jirovecii. History reveals the patient has experienced recurrent episodes of otitis media and oral ulcers since 4 months of age. Laboratory evaluation demonstrates profound neutropenia. Which of the following is the most definitive, rapid screening finding to support the suspected diagnosis?