For lack of capacity due to mental disease to be used as an affirmative defense, the mental illness must be diagnosed.

• A. True, diagnosis is required.
• B. Not required if the defendant had no prior offense.
• C. Diagnosis by any person is sufficient.
• D. Diagnosis is not relevant.

Answer: (A)

Lack of capacity due to mental disease as a defense relies on a diagnosed mental illness at the time of the offense. A formal diagnosis by a qualified professional provides the medical basis for claiming the defendant lacked the capacity to understand the act or to form the required intent. Without a diagnosis, the defense would rest on speculation rather than evidence, even if there is a history of mental illness. Diagnoses by laypersons aren’t typically sufficient, and factors like prior offenses don’t remove the need for a professional diagnosis.