The importance of explanations in decision-making, particularly in the medical domain, has been widely recognized. However, the evaluation of the quality of these explanations remains a challenging task. In this work, we propose a novel approach for assessing and evaluating the reasons provided in explanations about clinical cases. Our approach leverages an external knowledge base and a defined prevalence function to score each reason based on its pertinence in the domain. By applying a deterministic prevalence function, we ensure total transparency of the reasons' assessment, facilitating a precise explanation of the rationale behind the scoring hierarchy of each reason. We demonstrate the effectiveness of our approach in clinical cases, where medical experts explain the rationale behind a specific diagnosis and why other potential diagnoses are dismissed. Our methodology provides a nuanced and detailed evaluation of the explanation, contributing to a more comprehensive understanding of the decision-making process.