The doctor found that his joints were calcifying, which might cause mobility problems.

calcifying 是动词 calcify 的现在分词形式。calcify 由词根 calc-（表示"钙"）和后缀 -ify（表示"使...化"）组成，意思是"使钙化"或"变成钙质"。在医学上指组织因钙盐沉积而变硬的过程。