MAD SCIENCE

Metabolic bone disease :(  When reptiles lack sufficient UV lighting, vitamin D cannot be converted to its active form, and calcium cannot be sufficiently absorbed from their diets.  Instead the body resorts to nutritional secondary hyperparathyroidism, where parathyroid hormone stimulates release of calcium from the skeleton.  This keeps the blood’s calcium levels in the normal range, but leaves the reptiles with skeletons like rubber (hence why this bearded dragon’s bones are so gray on the radiograph, instead of bright white).  If you own a reptile, you need to either give it plenty of time outdoors in the sun, or make sure it has UVA and UVB spectrum lighting in its enclosure.

Edit: Thanks to nerdrvt, who pointed out that not all reptiles need UV lighting — nocturnal lizards and snakes, for example.  The best bet is to do extensive research before adopting an exotic pet of any kind, to make sure that you have the capabilities to support whatever requirements they have!