Dermatology MCQ - Infiltrative and Neoplastic Disorders - Mongolian spot

A healthy, full-term newborn is brought to the clinic for their first well-baby visit. The parents are concerned about a large, bluish-gray discoloration over the infant's lumbosacral area and buttocks. The lesion is flat, has a poorly defined border, and is completely asymptomatic. The rest of the physical examination is normal. What is the most accurate statement regarding the prognosis and management of this condition?

A. It is a sign of underlying spinal dysraphism and requires urgent ultrasonography or MRI for evaluation.
B. It represents a form of cutaneous melanoma with a high risk of metastasis, necessitating wide local excision.
C. It is a benign, self-limited dermal melanocytosis that will typically resolve spontaneously during early childhood.
D. It is a vascular malformation that may proliferate during infancy and require pulsed-dye laser therapy.
E. It is a persistent birthmark that will darken with age and has a significant association with metabolic storage diseases.

Correct Answer: C. It is a benign, self-limited dermal melanocytosis that will typically resolve spontaneously during early childhood.

Answer and Explanation

The correct answer is C. This question describes the classic presentation of a Mongolian spot (also known as congenital dermal melanocytosis). The key clinical clues are the presence at birth, the characteristic bluish-gray color, and the location over the lumbosacral area/buttocks. The most important fact for management is recognizing its benign and self-resolving nature.

Why the Other Options are Incorrect:

• A. It is a sign of underlying spinal dysraphism...: This is incorrect. While certain midline cutaneous lesions (e.g., tufts of hair, deep dimples, hemangiomas) can be associated with spinal dysraphism, a Mongolian spot is not. It is a purely cutaneous finding with no such association.

• B. It represents a form of cutaneous melanoma...: This is incorrect and a critical distinction. Mongolian spots are benign collections of melanocytes. Congenital melanoma is exceedingly rare and presents as a nodular, heterogeneous, and changing lesion, not a uniform, flat, bluish macule.

• D. It is a vascular malformation...: This is a common misdiagnosis based on color, but it is incorrect. The blue color of a Mongolian spot is due to the Tyndall effect (light scattering through the dermis), not deoxygenated blood. It is a melanocytic lesion, not a vascular one. Vascular malformations like a nevus of Ota appear in different distributions and do not resolve.

• E. It is a persistent birthmark that will darken with age...: This is incorrect. Mongolian spots typically fade and most resolve completely by age 3-5 years. Persistent spots into adulthood can occur but are not the norm. While extensive or atypical Mongolian spots can be associated with certain inborn errors of metabolism (e.g., GM1 gangliosidosis), this is rare, and an isolated, classic lumbosacral spot in a healthy infant has no such association.

Additional High-Yield Information for Exams:

• Histopathology: The hallmark is the presence of spindle-shaped melanocytes located deep in the dermis. These melanocytes are scattered between collagen bundles. This deep location is what gives the lesion its characteristic blue color.

• Differential Diagnosis: The main differential for a blue lesion in a newborn includes:

  • Mongolian Spot: As described; lumbosacral, resolves.

  • Nevus of Ota/Ito: Similar histology, but involves the first/second trigeminal nerve distribution (Ota) or shoulder/neck (Ito). These are typically persistent and do not resolve.

  • Blue Nevus: Usually a smaller, more circumscribed, blue-black papule. It is also persistent.

  • Bruise (Traumatic Ecchymosis): Will change color (green, yellow) and resolve over days to weeks, unlike a stable Mongolian spot.

• Prognosis: Excellent. The vast majority of classic Mongolian spots resolve spontaneously in early childhood without any intervention.

• Management & Rationale:

  • Rationale: The primary goal is to correctly diagnose and provide reassurance to the family, avoiding unnecessary investigations or anxiety.

  • First-line: Reassurance and education. Documenting the lesion in the medical record with a description or photograph is prudent to prevent future misidentification as a bruise, which could raise concerns for non-accidental injury.

  • No treatment is indicated.

  • Further Workup: Is only necessary if the presentation is atypical (e.g., extensive spots beyond the classic location, presence of other systemic abnormalities) which might prompt investigation for associated rare syndromes.