Dermatology MCQ - Infiltrative and Neoplastic Disorders - Speckled lentiginous naevus

A 5-year-old child is brought to clinic for evaluation of a "birthmark" on the leg. The mother explains that the child was born with a flat, light-brown patch, but over the last two years, multiple darker brown spots have appeared within it. On examination, you see a 6 cm x 4 cm, tan-colored macule on the calf, within which are numerous smaller, 2-5 mm, dark brown to black macules and papules. What is the most accurate histopathological description of the two components of this lesion?

A. The background patch shows a decreased number of melanocytes, while the dark speckles represent dermal nests of nevus cells.
B. The background patch shows features of a lentigo, while the dark speckles represent superimposed melanocytic nevi.
C. The entire lesion is a melanoma in situ, with the dark speckles representing foci of dermal microinvasion.
D. The background patch shows epidermal hyperplasia and horn pseudocysts, while the dark speckles represent foci of irritated seborrheic keratosis.
E. The background patch shows a vascular proliferation, while the dark speckles represent thrombosed capillaries.

Correct Answer: B. The background patch shows features of a lentigo, while the dark speckles represent superimposed melanocytic nevi.

Answer and Explanation

The correct answer is B. This question describes a classic Speckled Lentiginous Naevus (also known as a Nevus Spilus). This is a combined lesion. The background is a flat, tan patch that is, by definition, a lentigo (or sometimes a café-au-lait-like macule). The superimposed darker speckles are small, focal proliferations of melanocytes, which can histologically be junctional, compound, or dermal nevi, and occasionally blue nevi or Spitz nevi.

Why the Other Options are Incorrect:

• A. The background patch shows a decreased number of melanocytes...: This is incorrect. The background patch shows features of a lentigo, which is characterized by an increased or normal number of melanocytes, not a decreased one. The speckles are typically junctional or compound nests, not purely dermal.

• C. The entire lesion is a melanoma in situ...: This is incorrect and alarmist. While there is a very slightly increased risk of melanoma developing within a speckled lentiginous naevus over a lifetime, the lesion itself is benign. The description of a stable lesion developing over years in a child is classic for a benign process, not melanoma.

• D. The background patch shows epidermal hyperplasia and horn pseudocysts...: This describes a seborrheic keratosis. The background of a speckled lentiginous naevus is macular and shows lentiginous hyperplasia, not the verrucous architecture of a seborrheic keratosis.

• E. The background patch shows a vascular proliferation...: This describes a vascular lesion like a port-wine stain. The tan color indicates melanin, not blood.

Additional High-Yield Information for Exams:

• Histopathology:

  • Background Patch: Elongated rete ridges with basal layer hyperpigmentation. The number of melanocytes may be normal or increased, but they are not atypical and are singly dispersed.

  • Dark Speckles: These show nested proliferations of melanocytes. The specific type of nevus must be identified on histology (e.g., junctional nevus, compound nevus, blue nevus, Spitz nevus).

• Differential Diagnosis: The main differential is clinical recognition. It is distinct from:

  • Congenital Melanocytic Naevus: Usually uniformly pigmented and often more raised from the start.

  • Multiple Agminated Spitz Nevi: The speckles are all of one specific, often pink or tan, type.

  • Atypical Mole (Dysplastic Nevus) Syndrome: The nevi are scattered, not confined to a single background patch.

• Associated Conditions & Prognosis:

  • Benign Nature: The lesion itself is benign.

  • Malignant Potential: This is a key exam point. There is a very small but documented increased risk of melanoma developing within a speckled lentiginous naevus, likely arising from one of the speckled components. The lifetime risk is low but greater than that of an isolated common acquired nevus. Any rapid change, enlargement, or ulceration within a speckle should prompt biopsy.

• Management & Rationale:

  • Rationale: The goal is to avoid unnecessary excision of a often-large benign lesion while monitoring for the rare sign of malignant change.

  • First-line: Patient/parent education and clinical monitoring. Baseline clinical photographs are extremely helpful for tracking stability.

  • Biopsy: Is indicated for any single speckle that demonstrates atypical features (change in color, size, shape, or symptoms).

  • Excision: Prophylactic complete excision is generally not recommended due to the size and low absolute risk. However, it may be considered for lesions in difficult-to-monitor locations or if there is significant patient anxiety.