Mobilizing scientific knowledge for understanding the natural world and for critically appraise socio-scientific situations and make decisions are key competencies for today's' society. Therefore, it is essential to understand how students at the end of compulsory schooling use scientific knowledge for understanding the surrounding world. The objective of this study is to understand how students construct a scientifically oriented explanation while making sense of a phenomenon related to health. We analysed 526 answers to a question of a Portuguese national 9th grade science exam. The results reveal that most students present difficulties in explaining a phenomenon related to health supported by logical and clear reasoning, and in mobilizing scientific knowledge for connecting different scientific domains. We reaffirm the need to reconceptualize the role of health education within science education. Developing reading, writing and discussion activities related to health, which requires the use of scientific knowledge from different domains, would facilitate students' understanding and appropriation of scientific knowledge and its transference. This is a fundamental competence for promoting active and informed citizenship.