Dragons are a grand classic of fantasy. For this question, let's assume we're talking about the following stereotype:

• hatches from an ostrich-sized egg and can grow to mountain size if nothing limits it's growth (enough food and space)
• lizard-like appearance, bat like wings, able to fly
• breathes fire
• carnivorous

With an earth-like biology (or with believable variations), how close can we get of this dragon?

Explanations are especially needed for:

• fire breathing: how it works? how it happened through evolution? how it impacts the nutrition requirements (if gas is produced in high quantity, surely there is an impact there)? how is it not deadly for the dragon itself?
• flying: is the energy requirement believable (even with heavy fire-resistant scales)? how much can the body look like a komodo dragon without it being an aerodynamics issue?

And any other issue you can come up with ;-)