Set up a second table occurrence, Disease Classification 2, of Disease Classification. Link it to your patient info table by Disease.
In your value list specify that the values be listed from Disease Classification 2, Include only related values starting from Patient Info.
It may help to think of your conditional value list as values you want to display in a portal. "use values from" should be from this "portal's" table occurrence. The "starting from" table occurrence is the layout's table occurrence or one related to it that would enable this imaginary portal to display the same values you want to see in the conditional value list.
Works great! Thank you!