Which of the following is the hallmark pathophysiological finding that DIFFERENTIATES dengue haemorrhagic fever (DHF) from dengue fever (DF) according to WHO classification?

• A High-grade fever lasting more than 5 days
• B Thrombocytopenia with platelet count below 150,000/mm³
• C Haemorrhagic manifestations such as petechiae
• D Plasma leakage evidenced by haemoconcentration or serous effusions ✓

Explanation

Plasma leakage — manifested as haemoconcentration (haematocrit rise ≥20% above baseline or equivalent fall after fluid resuscitation) or serous effusions (pleural effusion, ascites, hypoproteinaemia) — is the cardinal feature that distinguishes DHF from dengue fever. Dengue fever also causes fever, thrombocytopenia, and haemorrhagic manifestations; these alone do not define DHF. All four WHO criteria (fever 2–7 days, haemorrhagic tendency, thrombocytopenia ≤100,000/mm³, and evidence of plasma leakage) must be present for DHF diagnosis.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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