Just to add a few details to the excellent comment above…
Vitamin D is required by all vertebrates for the absorption and metabolism of calcium. It is stored in the liver until required. For reptiles that are capable of absorbing D from what they eat, a diet of whole vertebrates, containing bone and liver, means they get enough vitamin D and do NOT need exposure to UVB light to make more. Snakes and varanids are in this category. Note that most varanids require a high basking temperature and if this is not provided, then digestion and absorption of required nutrients may be adversely affected.
Rickster,
I am assuming MBD was properly diagnosed by a vet for the varanid pictured with the bent spine. MBD is a range of conditions resulting in abnormal mineral metabolism affecting bone development or maintenance and there be can be a dozen different causes or more.
Sweetangel,
There is a range of reasons some people have unsatisfactory results using UVB lights. The UVB output degrades and these lights all need to be regularly replaced despite no loss of visual light intensity. You need to use an appropriately rated globe for the specific animal and it needs to be placed at the correct distance from the basking reptile’s skin to be effective. The reptile requires heat at the same time as the manufacture of vitamin D by photosynthesis occurs at its preferred body temperature. Where multiple animals are caged together, all need access to heat and UVB.
I do agree, however, that supervised exposure to nature sunlight is a great alternative if you can organise it several days a week for half an hour or more for those reptiles that do require high levels of regular UVB.
Blue
PS. The active form of vitamin D is called D3.