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. "Mr. Lee's Dilemma" is an abbreviated instructional case (i.e., a minicase) based on an assessment scenario in which PSTs are asked to attend to student reasoning about comparing the magnitude of fractions. PSTs are asked to judge the mathematical validity of students' explanations as a way of further developing their own MKT.