Background: Motivation matters in medical students' academic performance. However, few studies have specifically examined how motivation and external environmental factors (e.g., institutions) affect medical students' performance with large-scale data sets. The roles of selfefficacy and learning engagement in the mechanisms that govern how motivation affects academic performance are still unclear. Objective: This study aims to advance a comprehensive understanding about the relationships between medical students' motivation, self-efficacy, learning engagement, and academic performance in a nationwide survey, taking students' demographic factors and sociocultural environments into consideration. Design: We collected data from 1930 medical students in China. We probed the relations between studying variables. We then performed structural equation model (SEM) analysis to examine the mediating roles of self-efficacy and learning engagement on the relationship between motivation and academic performance. We further carried out multiple-group SEM analyses to compare differences between males and females, and between students in key universities and colleges (KUCs) and non-key universities and colleges (NKUCs). Results: Medical students in KUCs demonstrated significantly higher intrinsic motivation, better academic performance and lower extrinsic motivation than those in NKUCs. Male students reported higher intrinsic motivation but surprisingly lower academic performance than females. The total effect of intrinsic motivation on academic performance was larger than that of extrinsic motivation. There were significant indirect effects of either intrinsic or extrinsic motivation on academic performance through learning engagement. Besides, both intrinsic motivation and extrinsic motivation predicted self-efficacy; however, the direct effect of self-efficacy on academic performance was not significant. Conclusions: This study provided researchers with a holistic picture of students' types of motivation in relation to academic performance. Findings from this study can help in rethinking the role of self-efficacy in medicine, in finding more effective interventions for promoting medical students' levels of motivation, and in developing motivation-related counselling methods for different groups of medical students.

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It is well known that the incidence of postoperative cognitive dysfunction (POCD) is high in elderly patients. The pathogenesis and therapeutic mechanisms of POCD, however, have not yet been completely elucidated. The effects of gut microbiota, particularly in terms of regulating brain function, have gradually attracted increasing attention. In this study, we investigated the potential role of gut microbiota in POCD in aged male mice and attempted to determine whether alterations in gut microbiota would be helpful in the diagnosis of POCD. POCD and non-POCD mice were classified by hierarchical cluster analysis of behavioral results. Additionally, α- and β-diversity of gut microbiota showed a differential profile between the groups. In total, 24 gut bacteria were significantly altered in POCD mice compared with those in non-POCD mice, in which 13 gut bacteria were significantly correlated with escape latency in the Morris water maze test (MWMT). Remarkably, receiver operating characteristic curves revealed that the Dehalobacteriaceae family and Dehalobacterium genus are potentially important bacteria for the diagnosis of POCD. These findings indicate that alterations in the composition of gut microbiota are probably involved in the pathogenesis of POCD in aged mice. Novel therapeutic strategies regulating specific gut bacteria may be helpful for the prevention and treatment of POCD.