A 16-month-old otherwise healthy female presented with a 2-week history of rhinorrhea, cough, and tachypnea. Most recently she developed low-grade fevers to 100.5 °F, decreased wet diapers, and diarrhea for the past couple of days.

 

She was taken to her local emergency department for persistent symptoms, where she was noted to have increased work of breathing and hypoxemia necessitating supplemental oxygen.

 

Chest X-ray showed a right pneumothorax and right pleural effusion for which a needle decompression was performed. Initial complete blood count revealed a hemoglobin (Hgb) of 4.6 g/dL with a mean cell volume (MCV) of 117 fL and immature reticulocyte fraction of 75%.

 

Other notable initial labs included an elevated LDH at 2,242 u/L as well as an elevated AST at 160 u/L with a normal bilirubin level. Blood and pleural fluid cultures were obtained.

 

She was given a dose of ceftriaxone and vancomycin prior to admission to the intensive care unit for further management.

 

Based on the differential diagnosis, which of the following would be the next best step in your diagnostic workup?

 

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