**Question:** Selegiline acts by:

**Core Concept:** Selegiline is an irreversible monoamine oxidase-B (MAO-B) inhibitor used in the treatment of Parkinson's disease and depression. It selectively inhibits the enzyme MAO-B, which prevents the breakdown of neurotransmitters such as dopamine, norepinephrine, and serotonin, leading to increased availability of these neurotransmitters in the synaptic cleft.

**Why the Correct Answer is Right:** Selegiline is a selective inhibitor of MAO-B, which is responsible for the degradation of dopamine, norepinephrine, and serotonin. By inhibiting MAO-B, selegiline increases the amount of these neurotransmitters available for neuronal communication, thereby exerting its therapeutic effects in Parkinson's disease and depression.

**Why Each Wrong Option is Incorrect:**

A. MAO-A inhibition: Selegiline is a selective inhibitor of MAO-B, not MAO-A. Inhibition of MAO-A would lead to a different set of side effects and therapeutic outcomes.

B. Increased serotonin levels: While selegiline does increase the availability of serotonin, the primary action of selegiline lies in its inhibition of MAO-B.

C. Depression treatment by MAO-B inhibition: Although selegiline is used in the treatment of depression, the correct answer involves its MAO-B inhibition, not the treatment of depression itself.

D. Neuronal protection: While selegiline does exert neuroprotective effects, the primary action of selegiline is its MAO-B inhibition, which leads to increased neurotransmitter availability and therapeutic benefits.

**Clinical Pearl:** Selegiline's selectivity for MAO-B over MAO-A and its role in increasing neurotransmitter availability make it a valuable therapeutic agent in the treatment of Parkinson's disease and depression. However, it is essential to understand that the primary action of selegiline lies in its MAO-B inhibition, not the treatment of depression or neuronal protection.