What impact did the medical treatment have on the person's mental state?

The correct choice reflects that the medical treatment indeed had an influence on the person's mental state. This indicates that there was a change, whether positive, negative, or neutral, in how the individual felt or functioned mentally as a result of the treatment. The term "affected" captures the essence of the treatment making some sort of difference, which is often the case in medical contexts where interventions can lead to changes in mental well-being.

The other options do not convey this nuanced response effectively. For instance, "beneficial" would imply a positive change, which may or may not be true, as it does not acknowledge the possibility of negative or neutral impacts. "Neutral" suggests no effect at all, which contrasts with the notion of being "affected." Lastly, "disregarded" implies that the treatment was ignored or had no importance, which does not align with the premise of the question focusing on the impact on mental state.