In which odontogenic tumor is the EPITHELIAL component neoplastic while the mesenchymal component remains uninvolved (a reactive, non-neoplastic stroma)?

Correct answer: A. Squamous odontogenic tumor

• A. CORRECT. The SQUAMOUS ODONTOGENIC TUMOR is a benign neoplasm of odontogenic EPITHELIUM alone — islands of bland, well-differentiated squamous epithelium without peripheral palisading, set in a MATURE FIBROUS STROMA that is reactive rather than neoplastic. It arises from the rests of Malassez, appears as a triangular radiolucency lateral to the root of a vital tooth, and is treated by conservative excision or curettage.
• B. In ameloblastic fibrosarcoma BOTH components are abnormal, with the mesenchyme frankly malignant.
• C. Odontogenic myxoma is the reverse of what is asked: the MESENCHYMAL (ectomesenchymal) component is neoplastic, and odontogenic epithelium may be sparse or absent. It is locally infiltrative with a soap-bubble or tennis-racquet radiographic pattern.
• D. A compound odontoma is a HAMARTOMA, not a neoplasm — both epithelial and mesenchymal elements differentiate fully and form small tooth-like denticles. Neither component is neoplastic.

Note: The whole odontogenic tumor classification is built on this axis. EPITHELIAL only: ameloblastoma, CEOT, squamous odontogenic tumor, AOT. MESENCHYMAL only: odontogenic myxoma, odontogenic fibroma, cementoblastoma. MIXED: ameloblastic fibroma, ameloblastic fibro-odontoma, odontoma, odontoameloblastoma.