A 3-year-old presents with multiple poorly healing fractures, burn marks on his hands, and several missing teeth. His parents report he rarely cries with injury. He also has recurrent episodes of high fever, especially in warm weather, and his skin appears thick and calloused. This combination of congenital insensitivity to pain and anhidrosis (HSAN type IV) is caused by a loss-of-function variant in a gene encoding a receptor for what molecule?