The main of this article is to establish the elements of the Mathematics Teacher's Specialized Knowledge (MTSK) that are used to give timely attention to situations of affective domain in the classroom. We start from the assumption that to deal with this type of situation it is necessary to know the problem and its causes, but it also requires different knowledge that allows the teacher to deal with these situations effectively. Thus, a qualitative investigation was carried out from an instrumental case study. Regarding the results, it was found that indeed some elements of the MTSK are used to address affective situations in the classroom. For example, knowledge about these strategies, techniques, tasks and appropriate examples is used to promote motivation in students and the usefulness they attribute to mathematics.