What characterizes an incompetent adult?

An incompetent adult is characterized by having altered judgment or being under the influence of substances that impair their ability to think clearly and make rational decisions. This state can affect an individual's cognitive functions, hindering their capability to understand the implications of medical treatments or make informed choices regarding their health care. When someone is under the influence, they may not fully comprehend the consequences of their decisions or the information presented to them, leading to a lack of informed consent.

In contrast, the other options reflect characteristics of competence. A good understanding of treatment effects and the ability to make informed medical decisions are qualities typical of an adult who is deemed competent. Additionally, being a minor relates to age, not the competence level of an adult. Thus, the option regarding altered judgment accurately defines the condition of being incompetent.