Dermatology MCQ - Infiltrative and Neoplastic Disorders - Clinically atypical naevi

A 32-year-old patient with fair skin type presents for a skin examination. You note over 50 melanocytic nevi on their torso. One lesion on the upper back is 8 mm in diameter, with a flat, tan component and a slightly raised, darker brown center. The border is mildly irregular. The patient states it has looked this way for years. The patient's father had a melanoma diagnosed at age 50. What is the most accurate interpretation of this clinical scenario and the most important associated risk?

A. The described lesion is a seborrheic keratosis; the patient's risk for melanoma is solely related to their family history.
B. The described lesion is a clinically atypical naevus (dysplastic nevus); the patient has the atypical mole syndrome and is at a significantly increased lifetime risk for developing melanoma.
C. The described lesion is a congenital melanocytic nevus; the patient's risk for melanoma is confined to this specific lesion.
D. The described lesion is a superficial spreading melanoma; the patient requires immediate wide local excision.
E. The described lesion is a compound melanocytic nevus; the patient's numerous nevi indicate a slightly decreased risk for melanoma.

Correct Answer: B. The described lesion is a clinically atypical naevus (dysplastic nevus); the patient has the atypical mole syndrome and is at a significantly increased lifetime risk for developing melanoma.

Answer and Explanation

The correct answer is B. This question describes the classic presentation of a patient with the dysplastic nevus syndrome (atypical mole syndrome). The key features are:

• High total nevus count (>50)

• The presence of clinically atypical nevi (larger than 5mm, irregular borders, irregular pigmentation, and a "fried-egg" appearance with a flat periphery and raised center).

• A family history of melanoma.

This triad places the patient at a significantly increased lifetime risk for developing melanoma, which can arise from an existing atypical nevus or de novo on apparently normal skin.

Why the Other Options are Incorrect:

• A. The described lesion is a seborrheic keratosis...: This is incorrect. The lesion described has the "fried-egg" morphology characteristic of an atypical nevus, not the "stuck-on," waxy, verrucous surface with horn pseudocysts of a seborrheic keratosis. The risk is not solely from family history; the presence of atypical nevi is a major independent risk factor.

• C. The described lesion is a congenital melanocytic nevus...: This is incorrect. A congenital nevus is present at birth or appears in the first few months of life. The risk for melanoma, while increased, is not the patient's only risk; the syndrome itself confers a generalized risk.

• D. The described lesion is a superficial spreading melanoma...: This is a potential misdiagnosis, but it is incorrect based on the history of long-term stability. While melanoma must always be considered, a history of no change over years is strongly against a diagnosis of invasive melanoma. However, such a lesion still warrants close monitoring or biopsy to rule out melanoma in situ.

• E. The described lesion is a compound melanocytic nevus...: This is partially true but misses the critical point. While it is a compound nevus, its clinical features make it atypical. Furthermore, having numerous nevi is a marker of increased, not decreased, melanoma risk.

Additional High-Yield Information for Exams:

• Histopathology (if biopsied): An atypical/dysplastic nevus shows both architectural and cytological features:

  • Architectural disorder: Lentiginous melanocytic hyperplasia, bridging of rete ridges by nests of melanocytes, and a sparse dermal lymphocytic infiltrate ("shouldering").

  • Cytologic atypia: Melanocytes with enlarged, hyperchromatic nuclei of variable size and shape. Atypia is graded as mild, moderate, or severe.

• Differential Diagnosis: The primary and most critical differential is cutaneous melanoma. The "ABCDE" criteria (Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolution) were developed specifically to help distinguish atypical nevi from melanoma. Evolution (change) is the single most important factor.

• Associated Conditions & Prognosis:

  • Risk Stratification: The risk of melanoma is highest in individuals with a high nevus count, multiple atypical nevi, and a personal or family history of melanoma.

  • Prognosis: The atypical nevi themselves have a very low individual risk of malignant transformation. The greater danger is that they are a marker of a field defect, meaning the entire skin is at risk for developing melanoma elsewhere.

• Management & Rationale:

  • Rationale: The goal is lifelong surveillance for the early detection of melanoma, not the removal of all atypical nevi.

  • First-line: Total-body skin examinations every 6-12 months, depending on risk level. Sequential total-body photography and digital dermatoscopy are invaluable tools for tracking stability and detecting subtle changes.

  • Biopsy/Excision Indications: Any lesion that is changing (the "E" in ABCDE) or that stands out as markedly different from the patient's other nevi (the "Ugly Duckling" sign) should be biopsied. Prophylactic removal of all atypical nevi is not recommended.

  • Patient Education: Strict sun protection and monthly self-skin examinations are paramount.