The integration of basic science mechanistic knowledge (pathophysiology and etiology) with clinical features (signs and symptoms) during learning leads to robust cognitive representations in novices and supports the development of clinical reasoning, including better diagnostic accuracy and later learning of related concepts. However, previous studies have used a limited scope of traditional biomedical sciences, including biochemistry, anatomy, and physiology. The use of extended forms of foundational knowledge, including behavioral and sociological sciences, that have been proposed to support learning and performance in complex health systems remains unexplored. Thirty-three first-year medical students from the University of Toronto MD Program participated in the study. The effect of integrated extended basic science (EBS) learning was compared with that of clinically focused instruction on an initial assessment of diagnosis using clinical vignettes and a "preparation for future learning" assessment (PFLA) to assess learning of new related content in medical psychiatry (co-occurring physical and mental health conditions). Both forms of instruction supported the development of diagnostic ability on initial assessment (t[30] = 1.20, P = .24). On the PFLA, integrated instruction of extended forms of basic science led to superior performance on assessing complex patients' health care needs (t[30] = 2.70, P < .05). Similar to previous studies using integration of biomedical sciences, the integration of EBS can enhance later learning of new related concepts. These results have implications for curriculum design to support development of expert clinical reasoning.