Which medication is a selective MAOb inhibitor at low doses and a non-selective MAO inhibitor at high doses?

Selegiline is recognized for its unique pharmacological profile as a selective monoamine oxidase B (MAOb) inhibitor at low doses. This specificity enables it to primarily target the breakdown of dopamine, making it effective in treating conditions like Parkinson's disease without significantly impacting other neurotransmitters such as norepinephrine or serotonin, which are associated with mood disorders.

At higher doses, selegiline loses its selectivity and acts as a non-selective monoamine oxidase inhibitor (MAOI). This broader inhibition affects both MAO-A and MAO-B enzymes, thus influencing the metabolism of various neurotransmitters including norepinephrine and serotonin. This property allows it to address mood disorders, including depression, but also requires careful dietary restrictions to avoid hypertensive crises associated with higher levels of tyramine in certain foods.

The other options listed do not have this dual action where they operate as a selective MAOb inhibitor at low doses and non-selective at higher doses. For example, phenelzine, tranylcypromine, and isocarboxazid are non-selective MAO inhibitors regardless of dosage, affecting both MAO-A and MAO-B enzymes, which means they do not exhibit the same pharmacological flexibility as seleg