Final answer:
Erythema toxicum is a common benign rash that affects newborns. It is characterized by red, raised macular or papular lesions with a central point of clearing.
Step-by-step explanation:
When assessing a newborn boy at 12 hours of age, a nurse notes a rash on his abdomen and thighs composed of reddish macules, papules, and small vesicles. The nurse would document the finding as erythema toxicum. Erythema toxicum is a common benign rash that affects newborns and typically appears within the first few days of life. It is characterized by red, raised macular or papular lesions with a central point of clearing.