Ans: c (> 10 ppm) Ref: Maheswari, 3rd ed, p. 266; Davidson, 19th ed, p. 324Chronic flurosis is seen in localities where the water supply contains > 10 ppm fluride (Davidson).FlurosisIt is characterized by excessive deposition of calcium in bone & soft tissues.Clinical features:Dental flurosis-- earliest sign offlurosis (mottling of enamel) - incisors of upper jaw.Skeletal flurosis-- pain in back & joints, stiff spine, paraesthesias.Spastic paraparesis & anaemia.Radiological findings:Spine: Calcification of posterior longitudinal ligament.Pelvis: Calcification of ischio pubic & sacroiliac ligamentsForearm & leg: calcification of interosseous membrane.Treatment: Defluridation of water (Nalgonda technique)DD of generalized osteosclerotic lesions:FlurosisEngelman diseasePaget diseaseRenal osteodystrophyOsteopetrosis (marble bone disease)Secondaries from prostate and other organs.