Dengue hemorrhagic fever
Ans: C. Meningococcus
History of fever and headache for 3 days with blood pressure 90/60 mm Hg and rash on the legs are suggestive of meningococcemia.
The clinical-picture given in the question i.e. meningitis with a typical purpuric rash is seen in meningococcemia due to Neisseria meningitidis, which is Gram-negative diplococci and ferments glucose and maltose.
Gram-positive cocci catalase negative, bacitracin sensitive –> Streptococcus pyogenes
Gram-positive diplococci (lanceolate), catalase negative, optochin sensitive –> Streptococcus pneumoniae
Gram-negative diplococci, ferments glucose and maltose –> Neisseria meningitidis.
Gram-negative diplococci (kidney shaped), oxidase positive –> Neisseria gonorrhoea
Early symptoms are non-specific & suggest an influenza like illness with fever, headache & myalgia accompanied by vomiting & abdominal pain.