Dermatology MCQ - Infiltrative and Neoplastic Disorders - Seborrhoeic keratosis

A 65-year-old man presents with a concerning "mole" on his chest that has been slowly growing over the past few years. On examination, you find a 1.5 cm, well-demarcated, brownish-black plaque with a verrucous, "stuck-on" appearance and a waxy surface. When you gently scrape the surface with a blade, you note small, white, keratinous crumbs. The rest of his skin shows several similar, but smaller, lesions. What is the most likely diagnosis and the most appropriate next step?

A. Verruca vulgaris; treat with cryotherapy.
B. Nodular melanoma; perform an urgent excisional biopsy.
C. Seborrheic keratosis; provide reassurance and no further treatment is needed.
D. Actinic keratosis; treat with topical 5-fluorouracil or cryotherapy.
E. Pigmented basal cell carcinoma; perform a shave biopsy for diagnosis.

Correct Answer: C. Seborrheic keratosis; provide reassurance and no further treatment is needed.

Answer and Explanation

The correct answer is C. This question describes the classic presentation of a seborrheic keratosis. The key clinical clues are the older age of the patient, the "stuck-on" appearance, the verrucous/waxy surface, and the presence of similar lesions elsewhere (showing the "sign of Leser-Trélat" if eruptive and associated with internal malignancy, but typically just common, benign SKs). The description of "keratinous crumbs" with gentle scraping is a classic sign, representing the keratin-filled horn pseudocysts within the lesion.

Why the Other Options are Incorrect:

• A. Verruca vulgaris: While also verrucous, a common wart is caused by HPV and typically has a more irregular, cauliflower-like surface with black dots (thrombosed capillaries), not a waxy, stuck-on appearance. It does not produce keratinous crumbs in the same way.

• B. Nodular melanoma: This is the most critical entity to rule out. However, melanoma typically has a history of more rapid change, may ulcerate or bleed, and lacks the classic waxy, verrucous, and "stuck-on" quality. The dermoscopic appearance would be entirely different (homogeneous blue-black color, atypical vessels, etc.).

• D. Actinic keratosis: This is an irregular, scaly, erythematous macule or patch on sun-damaged skin, not a thick, waxy, papillomatous plaque. It is considered a pre-malignant lesion for squamous cell carcinoma.

• E. Pigmented basal cell carcinoma: This can be darkly pigmented but often has a pearly, rolled border and telangiectasias. It lacks the verrucous surface and keratinous cysts of a seborrheic keratosis. A shave biopsy would be diagnostic, but it is not necessary for a classic seborrheic keratosis.

Additional High-Yield Information for Exams:

• Histopathology: The hallmark features include:

  • Hyperkeratosis and papillomatosis (the verrucous surface).

  • Acanthosis (thickening of the epidermis).

  • Horn pseudocysts (round collections of keratin within the epidermis), which correspond to the clinical "keratinous crumbs."

  • The lesion is composed of basaloid keratinocytes and is entirely epidermal.

• Differential Diagnosis: The main differential, as outlined, is to distinguish it from melanoma, especially on a heavily pigmented or inflamed seborrheic keratosis. Other mimics include:

  • Stucco Keratosis: A variant of SK, lighter in color, and stuck-on like bits of stucco, typically on distal extremities.

  • Lentigo Maligna: A flat, irregularly pigmented macule, not raised or verrucous.

• Associated Conditions & Prognosis:

  • The Sign of Leser-Trélat: The sudden eruption of numerous, pruritic seborrheic keratoses can be a paraneoplastic sign associated with internal malignancies, most commonly gastrointestinal adenocarcinoma. However, this is rare, and the slow development of multiple SKs in an elderly person is overwhelmingly benign.

  • Prognosis: Excellent. Seborrheic keratoses are benign and have no malignant potential.

• Management & Rationale:

  • Rationale: The primary goal is accurate diagnosis and reassurance to avoid unnecessary procedures.

  • First-line: Clinical diagnosis and reassurance. No treatment is medically necessary.

  • Indications for Treatment: Treatment is elective and pursued for cosmetic reasons, or if the lesion is symptomatic (e.g., catches on clothing, becomes irritated or inflamed).

  • Treatment Options: Cryotherapy is the most common and effective method. Curettage (with or without electrocautery) is also highly effective. Ablative lasers can also be used.