Final answer:
The African American community is the marginalized group with the most frequently improperly diagnosed heart attack symptoms, as they face healthcare provider stigma and bias that lead to worse care.
Step-by-step explanation:
The marginalized group whose heart attack symptoms were MOST often improperly diagnosed was the African American community. Factors contributing to the misdiagnosis include stigma, lack of healthcare providers' awareness, and insensitivity toward the unique needs of various marginalized populations, which could lead to poorer health outcomes. Specifically, African Americans experience the higher incidence of several diseases such as hypertension, which is a major risk factor for heart disease, and are less likely to receive appropriate care due to their reporting of pain not being taken as seriously compared to their White counterparts.