Which of the following is known to cause vivid dreams as a side effect?

Sustiva, also known as efavirenz, is recognized for its potential to cause vivid dreams as a side effect. This drug is an antiretroviral medication used specifically in the treatment of HIV. Vivid dreams are associated with its action on the central nervous system, which can alter sleep patterns and dream recall. The mechanism by which Sustiva exerts these effects involves its influence on neurotransmitters and neurological pathways, leading to changes in the brain's sleep architecture, often characterized by increased dream intensity and recall.

In contrast, rifamycin is largely used as an antibiotic and is not typically associated with vivid dreams. Phenytoin, an anticonvulsant, primarily functions to stabilize nerve activity in the brain, while oxcarbazepine is another anticonvulsant with a different chemical structure and side effect profile that does not commonly include vivid dreams. Thus, the distinct impact of Sustiva on the brain's neurochemistry is what makes it notable for this particular side effect.