This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
Chordoma can occur over all the following sites, except - 
Select and explain. 
A. Rib, B. Clivus, C. Sacrum, D. Vertebral body
A: Rib
Answer: A: Rib 
Explanation: Chordoma occurs only in vertebrae and clivus 
Chordoma

Chordoma is a rare malignant tumour originating from the remnants of the primitive notochord.
As the vertebrae develop, notochord disappears and in adults, the normal remnant of notochord is nucleus pulpous of the intervertebral disc. In chordoma, this embryonic notochordal foci remains inside the vertebral body and forms tumours.
It occurs in adult males after 50 years.
It commonly occurs in the sacrococcygeal or in the sphere-occipital regions. The sacrum is the most common site: - Sacrum 50% clivus (35%), cervical thoracic/lumbar (15%).
Most of them occur in vertebral bodies.