In an 83-year-old Caucasian woman in good health with a 2‑month history of a 1‑cm dome‑shaped, firm, tense cystic lesion on the lateral right thigh that contains white proteinaceous material, a skin‑colored crescent, and a central 0.5‑cm punctum, what is the most likely diagnosis?

Most Likely Diagnosis: Inflamed Epidermoid Cyst

This lesion is most consistent with an inflamed epidermoid cyst (often erroneously called a "sebaceous cyst"), given the firm white proteinaceous material, cystic nature, and central punctum (the 0.5 cm SK-like appearance likely represents the follicular opening). 1

Clinical Reasoning

The key diagnostic features that point to this diagnosis include:

• Dome-shaped, firm, tense cystic lesion with white proteinaceous material visible through the skin
• Central punctum (what you described as a 0.5 cm SK) - this is the pathognomonic follicular opening/punctum of an epidermoid cyst
• "Cheesy keratinous material" extending below the surface - this is the classic content of epidermoid cysts 1
• Crescent of skin-colored area suggesting the cyst wall was partially visible through stretched overlying skin
• Frondly white firm proteinaceous material - characteristic of the keratin debris within epidermoid cysts

Important Clinical Context

Epidermoid cysts ordinarily contain skin flora in the cheesy keratinous material, even when uninflamed. When they become inflamed (as this one appears to be given the 2-month history and tense nature), cultures yield the same organisms, suggesting that inflammation occurs as a reaction to rupture of the cyst wall and extrusion of contents into the dermis, rather than as a true infectious complication 1.

Management Considerations

Your treatment approach was appropriate:

• Incision and evacuation is the correct treatment for inflamed epidermoid cysts 1
• The guideline explicitly states: "Simply covering the surgical site with a dry dressing is usually the easiest and most effective treatment of the wound" 1
• Gram stain, culture, and systemic antibiotics are rarely necessary unless there are unusual exceptions including multiple lesions, extensive surrounding cellulitis, or severe systemic manifestations 1

Common Pitfall to Avoid

The residual "frondly white firm proteinaceous material" you left behind is the cyst wall and remaining keratin debris. Complete excision of the cyst wall is ideal to prevent recurrence, but in the setting of acute inflammation, initial drainage followed by delayed complete excision (if recurrence occurs) is acceptable practice 1.

What Was NOT the Central Punctum

The structure you identified as a "0.5 cm SK" was almost certainly the follicular punctum (the opening of the cyst to the skin surface), not an actual seborrheic keratosis. This is a classic feature of epidermoid cysts and can appear as a dark or keratotic plug.

Follow-Up Recommendation

Monitor for recurrence. If the lesion reforms, consider formal excision of the entire cyst wall when inflammation has resolved. No antibiotics are needed unless signs of extensive cellulitis or systemic infection develop 1.