Dermatology MCQ - Infiltrative and Neoplastic Disorders - Comedones

A 17-year-old patient presents with numerous small, skin-colored lesions on their forehead and chin. The lesions are non-inflammatory and do not regress with the application of lateral pressure. A distinctive histopathological finding in these lesions is a dilated follicular infundibulum impacted with keratin and sebum. Which of the following best describes this type of lesion and its primary pathophysiological mechanism?

A. Closed comedones, primarily caused by the proliferation of Cutibacterium acnes leading to follicular rupture.
B. Open comedones, primarily caused by abnormal keratinocyte proliferation and cohesion within the follicular infundibulum.
C. Inflammatory papules, resulting from a host immune response to bacterial antigens.
D. Macrocomedones, caused by the use of topical corticosteroids leading to follicular dilation.

Correct Answer: B. Open comedones, primarily caused by abnormal keratinocyte proliferation and cohesion within the follicular infundibulum.

Answer and Explanation

The correct answer is B. This question describes the classic presentation of open comedones (blackheads). The key clinical clue is that they are "non-inflammatory and do not regress with lateral pressure," distinguishing them from other small papules. The description of a "dilated follicular infundibulum impacted with keratin and sebum" is the exact histopathology of a comedone. The primary pathophysiological event in comedone formation is abnormal follicular keratinization, where hyperproliferation and increased cohesion of keratinocytes in the infundibulum lead to the plug.

Why the Other Options are Incorrect:

• A. Closed comedones...: This is incorrect. The lesions described are open to the skin surface (the "blackhead" is oxidized keratin, not dirt). Closed comedones (whiteheads) appear as small, white, closed papules that do regress with lateral pressure. While C. acnes plays a role in inflammatory acne, it is not the primary cause of the initial comedone formation.

• C. Inflammatory papules...: This is incorrect. The question explicitly states the lesions are non-inflammatory. Inflammatory papules are red, tender bumps that represent the next stage in acne pathogenesis after comedones become inflamed.

• D. Macrocomedones...: This is incorrect. Macrocomedones are large, closed comedones, often greater than 1-2 mm. They are a specific subtype, often associated with treatment (like isotretinoin) or chronic sun exposure, and are not the typical lesion described here.

Additional High-Yield Information for Exams:

• Histopathology:

  • Open Comedone: A widely dilated follicular orifice filled with a keratinous plug. The "black" color is due to melanin oxidation and lipid oxidation, not dirt.

  • Closed Comedone: A follicular infundibulum that is dilated and blocked by a keratinous plug, but the opening to the skin surface is very narrow or closed.

• Pathophysiology (The Four Key Steps in Acne Vulgaris):

  1. Increased sebum production (influenced by androgens).

  2. Abnormal follicular keratinization (hyperproliferation and cohesion of keratinocytes leading to the microcomedo, the precursor to all acne lesions).

  3. Colonization with Cutibacterium acnes.

  4. Inflammation (both innate and adaptive immune responses).

• Differential Diagnosis: The main differential for non-inflammatory facial papules includes:

  • Comedones (as described)

  • Sebaceous Hyperplasia: Yellowish, umbilicated papules.

  • Favre-Racouchot Syndrome: Solar elastosis with large open comedones on the periorbital skin of older adults.

  • Trichostasis Spinulosa: Keratotic plugs containing multiple small vellus hairs, often on the nose.

• Management & Rationale:

  • Rationale: The goal is to normalize follicular keratinization and reduce impaction.

  • First-line Topical Therapy: Retinoids (tretinoin, adapalene, tazarotene) are the cornerstone of treatment. They work by normalizing the follicular keratinization process, preventing the formation of new comedones, and helping to expel existing ones.

  • Other Topical Agents: Salicylic acid (a comedolytic beta-hydroxy acid) and azelaic acid.

  • Procedural: Comedone extraction can provide immediate cosmetic improvement but does not address the underlying pathophysiology.

  • Patient Counseling: Emphasize that treatment is preventive and must be used consistently; results take 6-8 weeks. The black color is not dirt and cannot be washed away.