Ans: A (Metaphysis) Ref.ApleyS System of Orthopaedics and Fractures, 9th edition, pg. 199-200Explanation:OsteochondromaIt is one of the commonest 'tumours' of bone.It is a developmental lesion which starts as a small overgrowth of cartilage at the edge of the physeal plate and develops by endochondral ossification into a bony protuberance still covered by the cap of cartilage.Most common sites are the fast-growing ends of long bones and the crest of the ilium.In long bones, growth leaves the bump stranded further down the metaphysis.At the end of the normal growth period for that bone it stops enlarging.Any further enlargement after the end of the growth period is suggestive of malignant transformation.X-ray appearance is pathognomonic.There is a well-defined exostosis emerging from the metaphysis. its base co-extensive with the parent bone.It looks smaller than it feels because the cartilage cap is usually invisible on X-ray: however, large lesions undergo cartilage degeneration and calcification and then the X-ray shows the bony exostosis surrounded by clouds of calcified material.Multiple lesions may develop as part of a heritable disorder - hereditary multiple exostoses - in which there are also features of abnormal bone growth resulting in characteristic deformities.The incidence of malignant transformation is difficult to assess because troublesome lesions are so often removed before they show histological features of malignancy.Figures usually quoted are 1 per cent for solitary lesions and 6 per cent for multiple.Features suggestive of malignant change are:Enlargement of the cartilage cap in successive examinations.A bulky cartilage cap (more than 1 cm inthickness).Irregularly scattered flecks of calcification within the cartilage cap.Spread into the surrounding soft tissues.MRI may be needed to reveal these changes.