Clinical problem solving involves both diagnostic and predictive reasoning.Diagnostic reasoning is characterized by inference from observations to hypotheses; predictive reasoning, by inference from hypotheses to observations. We investigate the use of such strategiesby medical students at three levels of training in explaining the underlying pathophysiology of aclinical case. Our results show that without a sound, pre-existing disease classification, the use ofbasic biomedical knowledge interferes with diagnostic reasoning; however, with sound classification, biomedical knowledge facilitates both diagnostic and predictive reasoning.