Which of the following is NOT an MAOI?

• A. Phenelzine
• B. Tranylcypromine
• C. Selegiline
• D. Metformin

Answer: (D)

The key idea here is distinguishing drugs that inhibit monoamine oxidase from those that don’t. Monoamine oxidase inhibitors block the enzyme responsible for breaking down monoamines (like serotonin, norepinephrine, and dopamine), which is why they’re used in depression and some movement disorders. Phenelzine and tranylcypromine are classic non-selective, irreversible MAO inhibitors, and selegiline is a selective MAO-B inhibitor used for Parkinson’s disease. Metformin, in contrast, is a biguanide antihyperglycemic agent; it lowers glucose production in the liver and improves insulin sensitivity, and it does not inhibit monoamine oxidase. So Metformin is not an MAOI.