A PMHNP is evaluating available evidence to inform her clinical approach for patients with treatment-resistant depression. She identifies four sources of evidence during her literature review: a single-site randomized controlled trial with 45 participants, a Cochrane systematic review and meta-analysis of 12 RCTs, an expert consensus guideline published by a professional psychiatric organization, and a retrospective cohort study from a large academic medical center. When applying the hierarchy of evidence to inform her clinical decision-making, which source should the PMHNP prioritize as the highest level of evidence?