Responses like this are best entered into "Post A Answer" of the original "thread" where you asked your question so that people responding can see what posts preceded it and thus have a better understanding of your issues.
a) I'd guess that this is a conditional value list. See these threads to learn more about this method:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.
b) Presumably, this is a value list that specifies the "use values from field" option. you could:
1) put buttons labeled "all", "current", "discharge" next to your drop down that can be clicked instead of selecting a patient ID from the drop down.
2) define a calculation field in your table from which the value list draws its values with this expression: List ( PatientID ; "All" ; "Current" ; "Discharge" ). You'd then need to use a script--likely performed by the OnObjectSave script trigger--to check for those values in this field and to perform the correct action for the value selected.
c) Patient status is a field that should be defined in a Patient table where you have one record for each patient. Thus, changing the status in this one field changes the value for this patient for all parts of your system. I would guess that you have at least two tables, one for Patients and one for details about that patient that change with each time patient is seen by a medical professional and that the two tables are linked by a PatientID field in Patients. If so, you can put a status field from Patients on any layout based on the "detail" table and it will both show the current status of the patient and also can be edited to record a change in status.