A 11 years Male obese, developed pain at calves and back of knee joints, next day had patecheal rash on legs, soles, then on ABDOMEN and forearms, no other associated symptoms. Not responding to treatment..(...Prednisolone, antihistamines)... on 24 Feb had severe pain in ABDOMIN for one day only.

No changes in rash. CBC done in his TOWN is normal, BP 100/70 mmHg, pulse 78, no organomegaly. What can be the cause of this problem?