Dermoid Cyst — ORE Part 1 MCQ

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ModerateOral PathologyDermoid CystORE Part 1

A 50-year-old woman presents with a smooth, dome-shaped swelling in the midline of the floor of the mouth. It is firm, non-tender, and has been present since childhood. It does not transilluminate. What is the most likely diagnosis?

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Correct answer: A — Dermoid cyst

The diagnosis is dermoid cyst. The key clinical discriminator is the lack of transillumination. Dermoid cysts contain skin appendages (sebaceous glands, hair follicles) and are filled with keratin and sebum—these opaque contents prevent transillumination. In contrast, ranulas (salivary extravasation pseudocysts) present as translucent, blue swellings that typically transilluminate. Lymphangiomas are soft, fluctuant, and translucent, also transilluminating. Thyroglossal duct cysts typically move with swallowing or tongue protrusion (not mentioned here) and may transilluminate if fluid-filled. Epidermoid cysts lack skin appendages but still contain opaque keratin; they are less common in the floor of mouth at this age and presentation. The firm consistency, midline location, long history since childhood (slow growth pattern), and absence of transillumination are classic for dermoid cyst.

Reference: Dermoid Cyst of the Floor of the Mouth: Diagnostic Imaging Findings (NCB/NLM PMC5984270, 2018); Occurrence of dermoid cyst in the floor of the mouth: the importance of differential diagnosis in pediatric patients (PubMed 28678954, 2017), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5984270/