This class represents a question on the Patient Health Questionnaire (PHQ-9) Assessment Instrument. This abstract class exists primarily to re-define the generic AssessmentInstrument.item.question property to the PHQ9.item.question. In other words, of all the possible questions on all the possible assessment instruments, we are explicitly identifying the nine PHQ-9 questions. Note that because this class is abstract, implementers would likely not instantiate this class, but rather the nine concrete sub classes.