Which source provides the highest level of evidence according to the hierarchy of evidence?

Rationale: A. Case reports and case series are descriptive studies that focus on individual or small groups of patients. They lack control groups and randomization, making them low on the hierarchy of evidence due to limited ability to establish causation or generalizability. B. Systematic reviews and meta-analyses sit at the top of the hierarchy of evidence. They synthesize results from multiple high-quality studies, often randomized controlled trials, using rigorous and reproducible methods. This reduces bias and provides the strongest and most reliable evidence for guiding clinical practice. C. Randomized controlled trials (RCTs) are a high level of evidence and are considered the gold standard for testing interventions, but they rank below systematic reviews and meta-analyses because they represent evidence from a single study rather than a synthesis of many studies. D. Expert opinion is based on clinical experience rather than research evidence. While valuable in practice, it ranks lowest on the evidence hierarchy due to the high risk of bias and lack of systematic evaluation.