Ans. D: 20 petechiae per square inch Dengue fever and dengue hemorrhagic fever (DHF) are acute febrile diseases and caused by four closely related virus serotypes of the genus Flavivirus, family Flaviviridae. Dengue is transmitted to humans by the Aedes aegypti (rarely Aedes albopictus) mosquito, which feeds during the day. This infectious disease is manifested by a sudden onset of fever, with severe headache, muscle and joint pains (myalgias and ahralgias -- severe pain gives it the name break-bone fever or bonecrusher disease) and rashes. The dengue rash is characteristically bright red petechiae and usually appears first on the lower limbs and the chest; in some patients, it spreads to cover most of the body. The classic dengue fever lasts about six to seven days, with a smaller peak of fever at the trailing end of the disease (the so-called "biphasic pattern"). Cases of DHF also show higher fever, haemorrhagic phenomena, thrombocytopenia, and haemoconcentration. A small propoion of cases lead to dengue shock syndrome (DSS) which has a high moality rate. The classic picture is high fever with no localising source of infection, a petechial rash with thrombocytopenia and relative leukopenia. The WHO criteria of dengue haemorrhagic fever; all four criteria must be fulfilled: Fever, bladder problem, constant headaches, severe dizziness and loss of appetite. Hemorrhagic tendency (positive tourniquet test, spontaneous bruising, bleeding from mucosa, gingiva, injection sites, etc.; vomiting blood, or bloody diarrhea) Thrombocytopenia (3or estimated as less than 3 platelets per high power field) Evidence of plasma leakage (hematocrit more than 20% higher than expected, or drop in haematocrit of 20% or more from baseline following IV fluid, pleural effusion, ascites, hypoproteinemia) A tourniquet test/ Rumpel-Leede Capillary-Fragility Test/capillary fragility test determines capillary fragility. The test is defined by the WHO as one of the necessary requisites for diagnosis of Dengue fever. A Mood pressure cuff is applied and inflated to a point between the systolic and diastolic blood pressures for five minutes. The test is positive if there are more than 20 petechiae per square inch. This est does not have high specificity.