A 58-year-old woman with end-stage renal disease on hemodialysis for 6 years presents with severe pain in her thighs and lower abdomen. Examination reveals violaceous reticulated patches that have progressed to indurated plaques with a **central black necrotic eschar (marked B)** surrounded by livedo reticularis. She is on warfarin for atrial fibrillation and has been receiving calcium-based phosphate binders. Serum calcium is 10.2 mg/dL, phosphate 6.8 mg/dL, and PTH 850 pg/mL. Skin biopsy shows medial arteriolopathy with vascular calcification. Which of the following is the MOST important immediate intervention to prevent progression of the necrotic process marked **B**?