Which of the following best describes a mental illness?

A diagnosed condition impairing mental functioning is the most accurate description of mental illness. This definition encompasses a wide range of disorders that significantly affect an individual's thinking, feeling, behavior, and overall functioning. Mental illnesses can be chronic and can lead to difficulties in everyday life, such as managing relationships, holding a job, or even performing daily activities.

In contrast, an emotional response to stress may be part of the experience of someone with a mental illness, but it is not solely indicative of a mental disorder. Similarly, a temporary psychological state might describe a momentary experience of anxiety or sadness but does not meet the criteria for a mental illness, which requires a more persistent condition. Lastly, an assessment tool for practitioners refers to methods used to diagnose or evaluate mental disorders, rather than defining what a mental illness is. Thus, only the correct choice encapsulates the essence and lasting impact of mental health conditions as recognized in clinical practice.