Dermatology MCQ - Infiltrative and Neoplastic Disorders - Combined melanocytic naevi

A 35-year-old woman has a long-standing, stable lesion on her shoulder. A biopsy is performed. The pathology report describes a melanocytic lesion with two distinct populations of cells: large, epithelioid melanocytes with abundant cytoplasm forming nests in the superficial dermis, and a second population of smaller, spindled, heavily pigmented melanocytes located deeper in the reticular dermis. There is no significant cytologic atypia. What is the correct diagnosis?

A. A dysplastic nevus with architectural disorder and cytologic atypia.
B. A recurrent nevus phenomenon following an incomplete biopsy.
C. A metastatic melanoma to the skin from an unknown primary.
D. A combined nevus, specifically a Spitz nevus and a blue nevus.
E. A congenital melanocytic nevus with typical maturation.

Correct Answer: D. A combined nevus, specifically a Spitz nevus and a blue nevus.

Answer and Explanation

The correct answer is D. This question describes the classic histopathological definition of a combined melanocytic nevus. This is a lesion composed of two or more distinct populations of nevomelanocytes. The description of "large, epithelioid melanocytes" is characteristic of a Spitz nevus component, while the "smaller, spindled, heavily pigmented melanocytes" in the deep dermis are the hallmark of a blue nevus component. The lack of significant atypia supports the diagnosis of a benign combined nevus.

Why the Other Options are Incorrect:

• A. A dysplastic nevus...: While a dysplastic nevus shows architectural disorder (bridging of rete ridges) and cytologic atypia, it does not typically feature two distinctly different populations of melanocytes, such as the epithelioid and deeply pigmented spindled cells described.

• B. A recurrent nevus phenomenon...: This occurs when a nevus regrows after an incomplete excision. It is characterized by a proliferation of melanocytes confined to the epidermis and superficial dermis directly above a scar. It does not involve a second population of cells in the deep reticular dermis.

• C. A metastatic melanoma...: This is incorrect. Metastatic melanoma would show significant cytologic atypia, mitotic figures, and a pattern of infiltration that does not resemble the organized, benign components of a Spitz or blue nevus. The description of two benign cell populations is incompatible with metastasis.

• E. A congenital melanocytic nevus...: A congenital nevus shows a characteristic pattern of nevus cells extending between collagen bundles and around adnexal structures. While it can be combined with other types, the specific description of two distinct morphologies (epithelioid and spindled/blue) points more specifically to a combination of known nevus variants.

Additional High-Yield Information for Exams:

• Histopathology: This is the cornerstone of diagnosis. A combined nevus shows two or more distinct histologic patterns within a single lesion. Common combinations include:

  • Ordinary (common) nevus + Blue nevus (most common combination)

  • Spitz nevus + Blue nevus ("SPARK" nevus)

  • Deep penetrating nevus + Blue nevus

  • The components can be intermingled or exist side-by-side.

• Clinical Appearance: There is no single classic appearance. They can be asymmetric and have color variegation (e.g., a tan background with a dark blue-black focus), which can clinically mimic melanoma. This is why they are often biopsied.

• Differential Diagnosis: The main differential is melanoma. The presence of two different cell types can cause diagnostic difficulty. The key is to recognize that each individual component, in isolation, has benign characteristics. The lack of severe atypia, mitotic activity, and necrosis helps distinguish it from melanoma.

• Prognosis: Combined nevi are benign. There is no evidence that they have a higher malignant potential than their individual components. However, because the blue nevus component does not mature (the cells remain pigmented and spindled deep in the dermis), pathologists must be careful not to over-interpret this as a worrying feature.

• Management & Rationale:

  • Rationale: The goal is to achieve a definitive histological diagnosis to rule out melanoma, as the clinical appearance can be concerning.

  • First-line: Complete excision with narrow margins is both diagnostic and therapeutic. If a lesion has been partially biopsied and diagnosed as a benign combined nevus, but the clinical appearance remains concerning, complete excision is often recommended to ensure no atypical areas were missed.

  • Monitoring: If a lesion is not excised, close clinical monitoring is essential, though excision is generally preferred for diagnostically challenging lesions.