The first topic is vitamin A deficiency, mainly seen in turtles. This is a completely preventable disease, as it's related to improper feeding. The patient most commonly presents with swollen, puffy eyes and potentially torn or raw skin. It's also common to have some nasal discharge. Though lab tests can help establish the pet's health, this is mostly a diagnosis based on exam and clinical history.
Once the diagnosis is made, treatment is pretty simple. Improve nutrition! For carnivorous turtles I commonly recommend feeding a piece of liver once weekly. However, there are some other foods, especially for herbivorous animals. Here's a list of foods high in vitamin A:
Broccoli leaves and flowerets
Dandelion greens (beware of lawn treatments)
However, there is a danger in feeding supplements. Vitamin A can actually reach toxic levels, so you don't want to feed these vitamin-rich foods exclusively. Make sure to include them in the normal diet but don't go overboard.
Vitamin A deficiency is a perfect example of one of the main points of exotic pet care. A large majority of the health problems we see are due to inappropriate husbandry, housing, and diet. If you have these things well under control you'll be a long way to keeping your pet healthy.