An 18 year old boy presents with digital gangrene In third and fouh fingers for last 2 weeks. On examination the blood pressure is 170/110 mm of Hg and all peripheral pulses were palpable. Blood and urine examinations were unremarkable. Antinuclear antibodies, antibody to double stranded DNA and antineutrophil cytoplasmic antibody were negative. The most likely diagnosis is: