A 28-year-old man from Delhi presents with fever, myalgia, and headache on day 4 of illness. Laboratory work shows platelet count 85,000/μL, hematocrit 48% (baseline 42%), and AST 320 U/L. A second patient with dengue presents on day 2 of fever with similar symptoms but platelet count 150,000/μL, hematocrit 43%, and AST 180 U/L. Which finding best distinguishes the patient at risk for dengue hemorrhagic fever from one with uncomplicated dengue fever?