Dermatology MCQ - Infiltrative and Neoplastic Disorders - Epidermoid cysts

A 25-year-old man presents with a 2-cm, firm, subcutaneous nodule on his upper back. The lesion is mobile and non-tender. A tiny, central punctum is visible on the overlying skin. When expressed, a thick, cheesy, foul-smelling material is released. What is the fundamental pathological defect and the lining of this cyst?

A. A cyst originating from a ruptured hair follicle, lined by stratified squamous epithelium with a granular layer and filled with laminated keratin.
B. A cyst derived from the pilosebaceous unit, lined by a thin layer of cells resembling the outer root sheath and filled with homogeneous, oily material.
C. A cystic structure formed by the occlusion of an eccrine duct, lined by a double layer of cuboidal epithelium.
D. A developmental cyst due to trapped ectoderm along embryonic fusion lines, lined by stratified squamous epithelium and containing skin appendages.

Correct Answer: A. A cyst originating from a ruptured hair follicle, lined by stratified squamous epithelium with a granular layer and filled with laminated keratin.

Answer and Explanation

The correct answer is A. This question describes the classic presentation of an epidermoid cyst (formerly and often still called a sebaceous cyst, which is a misnomer). The key clinical clues are the location (sebaceous areas like the back, face, chest), the central punctum, and the character of the contents (cheesy, foul-smelling keratin). The fundamental pathology is that it is a true epithelial-lined cyst whose wall is identical to the epidermis, complete with a granular layer. It produces keratin, which fills the cyst.

Why the Other Options are Incorrect:

• B. A cyst derived from the pilosebaceous unit, lined by a thin layer of cells... and filled with homogeneous, oily material: This accurately describes a pilar cyst (trichilemmal cyst). Pilar cysts are most commonly found on the scalp, have a different lining (resembling the outer root sheath without a granular layer), and contain a more homogeneous, "motor-oil" like keratin.

• C. A cystic structure formed by the occlusion of an eccrine duct...: This describes an eccrine hidrocystoma, which is a thin-walled, translucent cyst that contains clear fluid, not cheesy keratin.

• D. A developmental cyst due to trapped ectoderm... containing skin appendages: This describes a dermoid cyst, which is a congenital cyst that often contains hair follicles, sebaceous glands, and sweat glands within its wall.

Additional High-Yield Information for Exams:

• Histopathology: The cyst wall is composed of stratified squamous epithelium with a well-formed granular layer. The cyst lumen is filled with layered (laminated) keratin. If the cyst ruptures, a significant foreign-body giant cell reaction to the extruded keratin is seen in the surrounding dermis, which is the cause of the inflammation, tenderness, and swelling in an "infected" cyst.

• Differential Diagnosis: The main differential includes:

  • Pilar Cyst (Trichilemmal Cyst): As described above; most common on the scalp.

  • Lipoma: A soft, lobulated mass of adipose tissue; no punctum.

  • Abscess: A painful, fluctuant collection of pus, often without a chronic history of a stable nodule.

• Associated Conditions & Prognosis:

  • Benign Nature: Epidermoid cysts are benign.

  • Complications: The most common complication is rupture and inflammation, often misdiagnosed as a bacterial infection. True bacterial infection can occur but is less common than sterile inflammation. Milia are tiny, superficial epidermoid cysts.

  • Rare Malignant Transformation: Very rarely, an epidermoid cyst can undergo malignant transformation into a squamous cell carcinoma, typically in older individuals in areas of chronic inflammation or radiation.

• Management & Rationale:

  • Rationale: For an asymptomatic cyst, the goal is often reassurance. For an inflamed or symptomatic cyst, the goal is to control inflammation and, if desired, achieve a definitive cure by removing the entire cyst wall to prevent recurrence.

  • Asymptomatic Cyst: Reassurance and observation. No treatment is necessary.

  • Inflamed (but not infected) Cyst: Intralesional corticosteroid injection (e.g., triamcinolone) is the first-line treatment to reduce inflammation and pain. Incision and drainage should be avoided as it does not remove the cyst wall and can lead to recurrence and scarring.

  • Definitive Treatment: Surgical excision of the entire cyst, including the wall, is the only definitive cure. This is best performed when the cyst is quiescent and not inflamed. The surgery involves a small elliptical incision to include the punctum and careful dissection to remove the cyst intact.