Urinary Tract Infections & Pyelonephritis — Topic Test

A 58-year-old man on warfarin for atrial fibrillation is started on an antibiotic for a urinary tract infection. Three days later he develops painful, sharply demarcated retiform purpura over the thighs and abdomen that rapidly turns into black necrotic eschars. Skin biopsy shows fibrin thrombi within dermal microvessels without vasculitis. Which protein deficiency, unmasked by this drug, best explains this complication?