There is a consensus that the items proposed by the Program for International Student Assessment (PISA) program allow us to focus on the outcomes of the processes of appropriation and transformation of learning tools at the end of compulsory schooling, particularly regarding the key competencies for lifelong learning and citizenship in digital societies. Taking into account these assumptions, this paper focuses on a fine-grained analysis of the dynamics of students' performance when they are confronted with a question from the mathematics domain in PISA 2012, through the example of the Climbing Mount Fuji item (question 1). In the context of the interaction dynamics (between a student and a research assistant), twelve 15-year-old students from Naples (Campania, Italy) were requested to think aloud when answering the question 1 of the item. Verbatim transcripts of the interactions are analyzed from the point of view of the PISA framework, the mathematical educational framework, and the socio-psychological approach based on didactic contract. The results show that the students involved in this task commit themselves in a complex reasoning, relying on mathematical requirements (e.g., different mathematical procedures) in an attempt to resolve ambiguities in the text, also referring to their everyday school life, activated by the didactic contract implied by the scenario of the question. The interweaving of PISA performances, mathematical procedures, and the socio-psychological approach to test assessment is discussed as a tool for a better understanding of teaching and learning activities.