A previously healthy 14-month-old girl presents for evaluation following the gradual appearance and persistence of multiple 4- to 15-mm dome-shaped yellow papulonodules on the scalp, face, and upper trunk. Skin biopsy reveals that the lesions are composed primarily of lipid-laden histiocytes, with scattered Touton-type giant cells and small admixtures of eosinophils and lymphocytes. Predominant cells stained strongly with factor XIIIa immunostain, but did not stain with S-100. The remainder of the physical examination was normal. Specifically, neither organomegaly nor lymphadenopathy were clinically detected.
Which test is most urgent in this situation? Pick the best choice.