Dermatology MCQ - Infiltrative and Neoplastic Disorders - Solar or actinic lentigo

A 58-year-old retired gardener presents for a skin check. He is concerned about a "brown spot" on his right cheek that has been present for several years. On examination, you find a 6 mm, well-circumscribed, uniformly light-brown macule on his malar eminence. The surrounding skin shows signs of chronic solar damage. Which of the following is the most accurate and definitive statement regarding the diagnosis and nature of this lesion?

A. It is a precursor to seborrheic keratosis and will likely become verrucous over time.
B. It is caused by a localized increase in the number of melanocytes, which is the sole driver of its appearance.
C. Its clinical presence is a strong independent predictor for the future development of malignant melanoma at that specific site.
D. It is characterized histologically by elongated rete ridges and hyperpigmentation of the basal layer.
E. Treatment with a potent topical corticosteroid is the first-line therapy to reduce its appearance.

Correct Answer: D. It is characterized histologically by elongated rete ridges and hyperpigmentation of the basal layer.

Answer and Explanation

The correct answer is D. This question describes a classic solar lentigo. The most accurate and definitive statement is one that captures its core pathological feature. Histologically, a solar lentigo is defined by elongated, club-shaped rete ridges and increased pigmentation in the basal keratinocytes.

Why the Other Options are Incorrect:

• A. It is a precursor to seborrheic keratosis and will likely become verrucous over time: This is incorrect. While some seborrheic keratoses may arise on a pre-existing solar lentigo (a lesion sometimes called a "lichenoid keratosis"), a solar lentigo itself is not a direct precursor and does not inevitably progress to a seborrheic keratosis. They are distinct entities.

• B. It is caused by a localized increase in the number of melanocytes, which is the sole driver of its appearance: This is partially correct but misleading. There is an increase in the number of melanocytes, but this is not the "sole driver." The hyperpigmentation is a result of both increased melanin production by melanocytes and increased transfer of melanin to the surrounding basal keratinocytes. The elongated rete ridges are a critical component of the pathology.

• C. Its clinical presence is a strong independent predictor for the future development of malignant melanoma at that specific site: This is incorrect and a critical distinction. A solar lentigo is a benign lesion and is not considered a direct precursor to melanoma. However, the presence of multiple solar lentigines is a marker of significant cumulative sun exposure, which increases a person's overall risk of developing skin cancers, including melanoma, elsewhere on the body. The lentigo itself has a very low malignant potential.

• E. Treatment with a potent topical corticosteroid is the first-line therapy to reduce its appearance: This is incorrect and could be harmful, leading to skin atrophy. Topical corticosteroids have no role in the treatment of solar lentigines.

Additional High-Yield Information for Exams:

• Histopathology: As stated, the hallmark is elongation of the rete ridges, which may appear club-shaped or bud-like. The ridges are often anastomotic. The basal layer shows hyperpigmentation. There is an increase in the number of melanocytes, but they are typically not atypical and are singly dispersed. The dermis shows solar elastosis.

• Differential Diagnosis: Key distinctions from:

  • Lentigo Maligna: Irregular borders and color; histology shows atypical melanocytes.

  • Junctional Nevus: Often darker brown; histology shows nests of nevus cells.

  • Ephelis (Freckle): Fades with reduced sun exposure; histology shows a normal number of melanocytes and normal rete ridges.

• Prognosis: Excellent. It is a benign lesion. The concern is primarily cosmetic.

• Management & Rationale:

  • Rationale: Reassurance is often sufficient. Treatment is elective and cosmetic.

  • First-line Options: Cryotherapy is very effective and commonly used. Topical therapies such as retinoids (tretinoin) or non-thermal light-based therapies (Q-switched lasers, intense pulsed light) are also excellent and can provide a more precise result with less risk of hypopigmentation.

  • Prevention: Strict sun protection with sunscreen and hats is paramount to prevent new lesions and darkening of existing ones.