X-ray of a young man shows hetrotopic calcification around bilateral knee joints. Next investigation would be

Serum phosphate This young man most probably has tumoral calcinosis Tumoral calcinosis Tumoral calcinosis is a rare condition which primarily, but not exclusively affects black people in otherwise good health. This disease usually presents in the second decade of lifeand is characterised by deposition of painless calcific masses around the hips, knee, elbow, shoulders, and gluteal. The primary defect responsible for this metastatic calcification appears to be hyperphosphatemiaresulting from the increased capacity of renal tabula and intestines to absorb phosphate. The most common laboratory findings are hyperphosphate,nia and elevated serum 1,25-dihydroxyvitamin-D levels. Serum calcium, parathyroid hormone and alkaline phosphatase levels are usually normal. Surgical excision is the most successful form of t/t, although recurrances are common. Medical tit to control the hyperphosphatemia (eg. a low phosphate diet and oral estimation of phosphate binders) is an imp. adjunct to surgical excision.