There is a broad consensus that beginning teachers of mathematics need a strong foundation in mathematical knowledge for teaching (MKT), defined as the mathematical knowledge required to recognize, understand, and respond to the mathematical work of teaching one must engage in. One recurrent challenge in teacher education is how to provide support for preservice teachers (PSTs) to acquire such competencies. Recent trends toward practice-based teacher education support the idea of engaging novice teachers in activities that are purposefully constrained to a core teaching practice. "Ms. Taylor's Choice" is an abbreviated instructional case (i.e., a minicase) based on an assessment scenario in which PSTs must select an example that will focus students' attention on how the associative and commutative properties can be used to evaluate expressions. PSTs are asked to take into account the instructional goals, the motivations of students, and the difficulty of the mathematics in choosing from among these examples as a way of further developing their own MKT.