A 7-year-old girl presents with a 3-day history of fever, malaise, and a bright red facial rash that appears as if she has been slapped on the cheeks. The rash spares the nasolabial folds and oral mucosa. Over the next 2 days, a lacy, reticular rash appears on her trunk and extremities that blanches with pressure. She has mild joint pain in her knees and wrists. Her mother reports that several children in her school developed similar illness 2 weeks ago. Full blood count shows mild anemia (Hb 10.2 g/dL) with reticulocytosis. What is the most likely diagnosis?