The other posts offer very usable solutions. A method I use requires a When event in "downstream" fields return a true or false based on input from an "upstream priority" field.
Example

... first a procedure to be called by downstream fields ..
Procedure OkayToSelectDownStreamField()
return "DR" $ upper(PriorityField.Value)

.. the downstream WHEN events look like this ..
return OkayToSelectDownStreamField()

>Hi. VFP6 user here. I am having trouble placing the user back in the current field when my data validation fails. Fox doesn't like SetFocus in the valid, and it doesn't seem to work for me in the LostFocus.
>
>For Example (in LostFocus)
>IF ! "Dr" $ This.Value
> =messagebox('This list should be Doctors only.',16,'Go Back')
> This.SetFocus
>ENDIF
>
>...doesn't work
>
>Thanks, Randy