Medical term for a stone formed in the body, such as a kidney stone or gallstone.
A stone was found in the kidney.
I had surgery for gallstones.
結核 (tuberculosis) and 結石 (a gallstone or kidney stone) share the kanji 結 and look similar, but they refer to completely different medical conditions.
Compound of 結 (けつ, 'bind' or 'form') and 石 (せき, 'stone'), literally 'formed stone'. The reading けっせき results from regular rendaku voicing of せき to ぜき.