Final answer:
Bupropion is the least likely to cause drug-drug interactions, making it a safer choice for an older patient who is taking numerous medications.
Step-by-step explanation:
Bupropion is the least likely to cause drug-drug interactions when compared to the other antidepressants mentioned. Bupropion is a norepinephrine-dopamine reuptake inhibitor, and its mechanism of action is different from the selective serotonin reuptake inhibitors (SSRIs) like citalopram, fluoxetine, paroxetine, and venlafaxine. This makes it less likely to interact with other medications that affect serotonergic systems.