Urza enters the battlefield, creating a 0/0 Construct artifact creature token.
Activate Urza's second ability by tapping the Construct, adding ({U} magic symbol) .
Mana Crypt resolves.
Activate Mana Crypt by tapping it, adding ({C} magic symbol) ({C} magic symbol) .
Activate Drafna's first ability by paying ({1} magic symbol) ({U} magic symbol) , returning Mana Crypt from the battlefield to your hand.
Repeat.
Once you have infinite colorless mana, you may get infinite blue mana by tapping Mana Crypt with Urza's ability, and you may activate Urza's last ability any number of times.