**Core Concept**
Dengue haemorrhagic fever (DHF) is a severe form of dengue infection characterized by plasma leakage, a significant increase in vascular permeability, and hemorrhagic manifestations. This condition is a result of an imbalance between the pro-inflammatory and anti-inflammatory responses to the dengue virus.

**Why the Correct Answer is Right**
The diagnosis of DHF is primarily clinical, supported by laboratory findings. The World Health Organization (WHO) criteria for DHF include a platelet count below 100,000 cells per microliter, a hematocrit increase of more than 20% from baseline, and evidence of plasma leakage or hemorrhage. The presence of the dengue virus can be confirmed through reverse transcription-polymerase chain reaction (RT-PCR) or serological tests, such as enzyme-linked immunosorbent assay (ELISA).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but it may refer to a specific laboratory test or clinical criterion that is not a primary diagnostic measure for DHF.
* **Option B:** This option may refer to a specific clinical presentation or symptom, but it is not a comprehensive diagnostic criterion for DHF.
* **Option C:** This option may refer to a specific laboratory finding, but it is not a primary diagnostic measure for DHF.

**Clinical Pearl / High-Yield Fact**
Dengue hemorrhagic fever is a medical emergency that requires prompt recognition and management to prevent complications and mortality. The WHO criteria for DHF should be used in conjunction with clinical judgment to guide diagnosis and treatment.

**Correct Answer: D. WHO criteria for DHF**

✓ Correct Answer: D. Acute onset of high fever, positive tourniquet test, bleeding gum and platelet count less than 1.0lakh