This class represents an answer or a response to a question on the Quality of Life Questionnaire for Dialysis Patients Assessment Instrument. This abstract class exists primarily to re-define the generic AssessmentEvent.responseItem.answer property to the DialysisPatientQOL.responseItem.answer. In other words, of all the possible answers on all the possible kinds of assessment instruments, we are explicitly identifying the possible answers to the sixteen DialysisPatientQOL questions. Note that because this class is abstract, implementers would likely not instantiate this class, but rather the sixteen concrete sub classes.