Dermatology MCQ - Infiltrative and Neoplastic Disorders - Lichenoid keratosis

A 55-year-old woman presents with a solitary, slightly violaceous, 6-mm flat-topped papule on her forearm. The lesion appeared several months ago and is mildly pruritic. She is concerned it might be a new mole. A dermatoscope is used and reveals a background of coarse, granular, gray dots. What is the most accurate description of the pathogenesis and histological findings of this lesion?

A. It is an actinic keratosis undergoing regression; histology shows parakeratosis and basal layer dysplasia.
B. It is an inflamed seborrheic keratosis; histology shows an irritated SK with squamous eddies.
C. It is a lichenoid keratosis, representing an immune-mediated regression of a pre-existing benign keratinocytic lesion; histology shows a lichenoid infiltrate with cytoid bodies.
D. It is a lichen planus; histology shows a saw-tooth acanthosis and a dense band-like lymphocytic infiltrate.
E. It is a lentigo maligna; histology shows atypical melanocytes proliferating along the basal layer.

Correct Answer: C. It is a lichenoid keratosis, representing an immune-mediated regression of a pre-existing benign keratinocytic lesion; histology shows a lichenoid infiltrate with cytoid bodies.

Answer and Explanation

The correct answer is C. This question describes the classic presentation of a lichenoid keratosis (also known as benign lichenoid keratosis or BLK). The key clues are the solitary nature, the location on sun-exposed skin (forearm), the violaceous color, and the dermatoscopic finding of coarse, granular, gray dots (which represent melanophages in the dermis, a sign of regression). The fundamental pathogenesis is an immune-mediated attack that is destroying a pre-existing lesion, most commonly a seborrheic keratosis or a solar lentigo.

Why the Other Options are Incorrect:

• A. It is an actinic keratosis undergoing regression: While a regressing actinic keratosis can have a lichenoid tissue reaction, the clinical presentation of a solitary, violaceous, flat-topped papule is more classic for a lichenoid keratosis. The history is less suggestive of a scaly, pre-malignant lesion.

• B. It is an inflamed seborrheic keratosis: An "irritated" seborrheic keratosis typically shows more epidermal hyperplasia and squamous eddies histologically, not the dominant lichenoid (band-like) inflammatory infiltrate characteristic of a lichenoid keratosis. The dermatoscopic gray dots are a sign of regression, not irritation.

• D. It is a lichen planus: This is the most important clinical mimic. However, lichen planus is typically multifocal, intensely pruritic, and often involves mucous membranes. A solitary lesion on the forearm would be an unusual presentation for classic lichen planus. Histologically, they can be identical, making clinical correlation essential.

• E. It is a lentigo maligna: Lentigo maligna presents as a slowly enlarging, irregularly pigmented macule on chronically sun-damaged skin. It is not typically a violaceous, flat-topped papule. The dermatoscopy of lentigo maligna shows an asymmetric pigmented follicular opening and rhomboidal structures, not the granular gray dots of regression seen in lichenoid keratosis.

Additional High-Yield Information for Exams:

• Histopathology: The histology is characterized by:

  • A dense, band-like (lichenoid) lymphocytic infiltrate in the upper dermis, obscuring the dermo-epidermal junction.

  • Vacuolar degeneration of the basal layer.

  • Necrotic/apoptotic keratinocytes (cytoid bodies, Civatte bodies) within the epidermis.

  • Melanophages in the papillary dermis (correlating with the dermatoscopic gray dots).

  • Remnants of a pre-existing lesion (e.g., a solar lentigo or seborrheic keratosis) may be seen at the periphery.

• Differential Diagnosis: The primary differential is with lichen planus and superficial basal cell carcinoma. The key is that lichenoid keratosis is almost always solitary.

• Associated Conditions & Prognosis:

  • Benign Nature: Lichenoid keratosis is a benign inflammatory lesion. It is not a precursor to malignancy.

  • Natural History: It often undergoes spontaneous regression over months to years, sometimes leaving behind a hypopigmented macule.

• Management & Rationale:

  • Rationale: The goal is to confirm the diagnosis and rule out malignancy, as it can clinically mimic basal cell carcinoma and melanoma.

  • First-line: For a classic lesion, clinical and dermatoscopic monitoring is acceptable. However, due to diagnostic uncertainty, a biopsy (often a shave biopsy) is frequently performed to confirm the diagnosis.

  • Treatment: If symptomatic (pruritic) or for cosmetic reasons, treatment options include mid-potency topical corticosteroids to reduce inflammation or cryotherapy. If biopsied and diagnosed, no further treatment is necessary as it will likely regress on its own.