The correct answer is Mongolian spot (choice “a”), which is seen in about 90% of newborn Native Americans. True ecchymosis (choice “b”) would not persist beyond a few days at most. Blue nevus (choice “c”) is still possible, but unlikely given the archetypical presentation and commonality of this Mongolian spot. If serious consideration were to be given to either blue nevus or nevus of Ito (choice “d”), biopsy would have to be done. However, the latter is quite unusual, while the former is usually much smaller than the patient’s lesion.
The Mongolian spot, also known as congenital dermal melanocytosis, usually presents either at birth or shortly thereafter and disappears by age 4. It only involves the skin and results from entrapment of melanocytes in the dermis during their migration from the neural crest to the epidermis. This case illustrates a typical location, but Mongolian spots can present as multiple lesions, usually on the posterior trunk, or as one large (> 20 cm) lesion. Fortunately, they are not associated with significant mortality or morbidity.
In addition to its high prevalence in Native Americans, Mongolian spots are seen on approximately 80% of Asian and 70% of Hispanic newborns, but less than 10% of white babies.
Blue nevi are quite common, representing a type of melanocytic nevus that is totally benign. Their color is actually brown, but these lesions develop relatively deep in the dermis and when viewed through the skin, appear blue—hence the name. Blue nevi, typically much smaller than our patient’s Mongolian spot, are often removed to rule out melanoma, which they can resemble at times.
The rare nevus of Ito represents a benign dermal melanocytic condition. It most commonly affects the shoulder.