The dentist, the supermarket, the park, etc. Because you are referring to a specific one of each, even if a listener doesn’t necessarily know which specific on

The dentist, the supermarket, the park, etc. Because you are referring to a specific one of each, even if a listener doesn’t necessarily know which specific one, they know that you know. In the case of your example the listener assumes you mean whichever specific dentist you are currently seeing. That fact that you may choose one randomly each time is an exception.