A 9-yeаr-оld Asiаn child is brоught tо the clinic by his pаrents for a school physical. On examination, the nurse practitioner notes several circular, purple ecchymosis‑like marks of varying sizes across the child's upper back. The child denies pain, and his parents explain that the marks appeared after they treated him for a recent cold. He appears well and shows no signs of distress or additional bruising on exam. Which of the following is the most likely explanation for these findings?