Which cyst tends to occur on the scalp as a movable subcutaneous nodule with firmer keratin content and may pop out during surgery?

• A. Epidermal inclusion cyst
• B. Lipoma
• C. Dermatofibroma
• D. Pilar cysts

Answer: (D)

Scalp cysts that present as smooth, mobile nodules under the skin and contain firm keratin point to pilar cysts (trichilemmal cysts). These arise from the hair follicle isthmus and are packed with dense keratin, which makes them feel firmer than other cysts. Because of their keratin content, the contents can sometimes be expressed or pop out during surgical removal, which is a classic intraoperative occurrence.

Epidermal inclusion cysts can also contain keratin but are more common on the face and trunk, often have a visible punctum, and tend to be softer. Lipomas are soft, fatty, and not keratin-filled, while dermatofibromas are firm nodules usually on the extremities and lack keratin content.