A 42-year-old man with a 10-year history of psoriasis vulgaris presents with multiple well-demarcated erythematous plaques on his elbows and knees covered with silvery-white scale. On examination, when the examiner gently scrapes away the scale from one of the plaques, pinpoint bleeding is observed at the site. The finding marked **D** in the diagram represents this clinical sign. Which of the following best explains the pathophysiology underlying this finding?