What is the most characteristic histologic finding on a bone marrow biopsy for a suspected case related to an infant with elevated urinary HVA/VMA?

The correct response identifies small round blue cells with dense nuclei forming small rosettes as the most characteristic histologic finding associated with an infant exhibiting elevated urinary HVA (homovanillic acid) and VMA (vanillylmandelic acid). These metabolites are associated with neuroblastoma, a common pediatric tumor that typically arises from neural crest cells.

In the context of neuroblastoma, histologic examination of the bone marrow may reveal the presence of these characteristic small round blue cells. The term "small round blue cells" refers to the appearance of the tumor cells under the microscope, where they display high nuclear-to-cytoplasmic ratios and delicate cytoplasm. The formation of rosettes is particularly significant, as it indicates a neuronal or neuroblastic differentiation of the cells. This feature helps distinguish neuroblastoma from other conditions that may cause similar clinical symptoms.

The presence of these cellular features aligns with the diagnosis and is supported by the increased levels of HVA/VMA, which are byproducts of catecholamine metabolism produced by neuroblastoma cells. This combination of clinical and histological findings is pivotal for establishing the diagnosis and guiding further management of the patient.