Which antidepressant drug is a selective monoamine oxidase-B inhibitor?

Selegiline is indeed classified as a selective monoamine oxidase-B (MAO-B) inhibitor. This particular type of medication primarily inhibits the MAO-B isoenzyme, which is primarily involved in the breakdown of dopamine. By selectively inhibiting MAO-B, selegiline helps to increase the levels of dopamine in the brain, making it particularly effective in the treatment of Parkinson's disease and sometimes depression, especially in patients who may benefit from enhanced dopaminergic function.

The other options listed do not fit this classification. Phenelzine is a non-selective monoamine oxidase inhibitor, affecting both MAO-A and MAO-B, while amitriptyline and escitalopram are both antidepressants that belong to different classes of medications (tricyclic antidepressants and selective serotonin reuptake inhibitors, respectively). That specificity in action and indication is what distinguishes selegiline as the correct answer in the context of MAO-B inhibitors.