Q. A 16-year-old boy is brought to the dermatologist by his mother, who is concerned about multiple small, dark red spots that have been appearing on his body over the past few months. The spots are painless and mostly located in the groin, umbilical region, and thighs. She also reports that her son seems to sweat less than his peers during sports activities and has had episodes of sharp pain in his hands and feet that come and go. The family history is notable for early cardiovascular disease and kidney problems. Physical examination confirms the presence of numerous non-blanching, red-purple macules. Neurological examination is unremarkable. Given this patient's symptoms, family history, and physical examination findings, what is the most likely diagnosis?