By which measure are you trying to find patients?
I think you may need to remember that MDX is not SQL although it can look similar and that cubes do not store or display data in the same method as a RDBMS (Such as SQL). the data you look for in a cube is a slice by 3 dimensions, rather than a 2 dimensional flat model like an RDBMS.
The measure is your way of getting dimensions with certain data.
In principle, your MDX will be along the lines of:
[Measure].[your measure] ON Rows
, [Patient].[PatientID]., [Patient],[PatientName].MEMBER ..etc ON Columns
But I would definitely suggest picking up an MDX reference book if you are new to it.