You are answering a USMLE-style medical question. Question: A 4-year-old boy is brought to the physician because of a generalized rash for 3 days. During this period, he has had severe itching and was not able to sleep well. He has also had fever, headache, and some muscle pain. Five days ago, he had a runny nose which subsided with over-the-counter medications. He returned from a camping trip 1 week ago. He attends a daycare center. The patient is at the 55th percentile for height and at the 50th percentile for weight. His temperature is 38.2°C (100.7°F), pulse is 97/min, and blood pressure is 96/60 mm Hg. Examination of the skin shows several macules, papules, and crusted lesions over his face, trunk, and extremities. There are a few fluid-filled vesicles over his abdomen and back. There is no cervical lymphadenopathy. The remainder of the examination shows no abnormalities. Which of the following is the most likely diagnosis? Options: A) Measles B) Urushiol-induced dermatitis C) Chickenpox D) Rubella Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.