A 2-year-old boy presents with a high fever of 39.4°C (103°F), splenomegaly, and pancytopenia (hemoglobin 8.5 g/dL, platelets 70 x 10³/mL, neutrophils 0.8 x 10³/mL). Laboratory studies are significant for a ferritin level of 8,000 ng/mL and hypofibrinogenemia. According to the Histiocyte Society diagnostic criteria, which of the following additional findings would be most supportive of the most likely diagnosis?