This exploratory study aims to contribute to the limited literature regarding the characteristics of STEM gatekeeper courses (Seymour & Hewitt, 1997; Suresh, 2006; Weston et al., 2019; Vyas & Reid, 2023) and their role in student retention and attrition by centering issues of ableism and racism (Annamma et al., 2013). I argue that gatekeeper courses, through their use of high-stakes assessment, reinforce experiences of ableism and racism that depress students' aspirations toward STEM baccalaureate degree attainment (Riegle-Crumb, King, & Irizarry, 2019; Pfeifer et al., 2021; Vyas & Reid, 2023).To better understand how these courses exacerbate systemic ableism and racism, we need to identify the STEM gatekeeper courses, determine commonalities in the courses' student assessment practices, and analyze course outcomes for students with intersectional dis/ability and minoritized racial identities. To do this, I posed the following research questions, given the setting is a research-focused science and technology university: 1) What courses are identified as undergraduate STEM gatekeeper courses? 2) What common assessment practices used to assess student learning are found across undergraduate STEM gatekeeper courses? 3) To what extent do STEM gatekeeper course assessment practices exacerbate the impacts of ableism and racism on student DFWI rates? In this study, I collected enrollment, course outcome, and demographic data from Fall 2019 to Spring 2021, excluding Spring and Summer 2020, at a research-focused science and technology university. Across this time period, a total of 92,501 students were enrolled in the 262 identified STEM gatekeeper courses. The STEM gatekeeper courses identified in this study extend into upper-level courses (e.g., 300-level courses) rather than strictly introductory-level STEM courses. To summarize the assessment practices implemented in the identified courses, I collected and analyzed 173 STEM gatekeeper course syllabi. Assessment constructs that were not captured nor summarized within the current education student assessment literature became apparent (e.g., students were allowed to use other resources during exams, the number of allowed assessment attempts varied, and the frequency assessments were disseminated varied widely). From these constructs, I developed a new categorization of assessment practices. This new categorization ranks assessments based on a "student-centric" to "instructor-centric" continuum. The STEM gatekeeper courses at this institution had an average "student-centric" to "instructor-centric" score of 11.58 on the five to 14 assessment scale, which is more "instructor-centric. "Overall, this study operationalized a Quantitative Critical (QuantCrit) theoretical framework using a Dis/ability Critical Race Studies (DisCrit) lens. Further, I developed and implemented a Bayesian methodological framework that better aligns with the tenets of QuantCrit with a DisCrit lens. This allowed me to construct a Bayesian logistic hierarchical linear model to support answering the third research question. From this model, I found that the institution's response to over usage of "instructor-centric" assessments in STEM gatekeeper courses through retrofitting accommodations -- an ableist practice -- has perpetuated systemic racism. Future research should focus on the construction of faculty development programs to support implementation of equitable assessment practices like choice in assessment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]