A 32-year-old woman from Mumbai presents on day 5 of fever with severe thrombocytopenia (platelets 20,000/μL), petechial rash, and spontaneous bleeding. Her friend, also infected with the same virus, presents on day 3 of fever with high fever, myalgia, and headache but normal platelet count and no bleeding manifestations. Which clinical feature best distinguishes dengue hemorrhagic fever (DHF) from dengue fever (DF)?