Derm Dx: Bluish-black discoloration of the face

A 53-year-old woman without significant medical history presented with darkening of the skin on her face. She has a history of melasma and has been used a variety of skin lightening products from Mexico and Central America. Examination reveals bluish-black macules on the zygomatic regions of her face. A skin biopsy demonstrates a glassy yellow-brown substance deposited in the dermis. ...