A 39-year-old woman with a long-standing vascular malformation of the posterior tongue underwent intralesional sclerotherapy with a single dose of bleomycin. Within 1 week after treatment, painless, nonpruritic, flagellate hyperpigmentation developed on her trunk. She had no alopecia and no changes in the appearance of nails or mucosa. She reported having no dyspnea or pulmonary symptoms.
What is the reason for this condition ?