You cannot. The display value is a pure view notion and view cannot be accessed by neither rules not scripting.
Your workaround is to define an aux data source based off an XML file where you will type the values for the dropdown. For example, it may look like this:
<data>
<option value="0">Patient</option>
<option value="1">Nurse</option>
...
</data>
Now you can bind your dropdown to this data source instead of typing the data in manually. Then when you look at /dfs:myFields/my:ComboReason, you can look up this value in the data source for the aux dom and find your string there.
A bit of an effort, but it works!
Good luck!
Dave
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge