Field placements serve as the traditional "clinical" experience for prospective mathematics teachers to immerse themselves in the mathematical challenges of students. This article reports data from a different type of learning experience, that of a clinical simulation with a standardized individual. We begin with a brief background on medical education's long-standing use of standardized patients, and the recent diffusion of clinical simulations to teacher and school leader preparation contexts. Then, we describe a single mathematics simulation and report data from prospective mathematics teachers' interactions with a standardized student on the issue of iconic interpretation. Findings highlight teachers' diagnostic, explanatory, mathematical, and instructional repertoires, as they guide a standardized student through two different graphing problems. Implications focus on the trends in teachers' instructional decisions, contextualized explanations, and the use of clinical simulations to enhance mathematics teacher development.