A 26-year-old woman with a metastatic tumor shows high levels of desmin in biopsy. Where likely did this tumor originate?

• A. Bone
• B. Epithelial
• C. Muscle
• D. Nerve

Answer: (C)

Desmin is a key intermediate filament protein that is specifically expressed in muscle tissues, including both skeletal and cardiac muscle. When high levels of desmin are detected in a biopsy, it strongly indicates that the tumor is of muscular origin, as desmin serves as a marker for muscle differentiation.

Given that the biopsy in question shows elevated desmin levels and is associated with a metastatic tumor, it is most consistent with a rhabdomyosarcoma or another type of soft tissue sarcoma that arises from muscle tissue. Tumors arising from other tissues, such as epithelial (which would express keratin), nerve (which might involve neurofilament proteins), or bone (which might show osteocalcin or other bone-specific markers), would not exhibit significant levels of desmin. Thus, the presence of high levels of desmin directs us towards a tumor that originated from muscle.