Problem-based learning promotes causal physiological reasoning in first-year medical students before formal cardiovascular instruction.

Teaching Human Physiology in undergraduate medical education requires more than the acquisition of conceptual knowledge; it demands that students develop causal explanatory frameworks linking physiological mechanisms to their clinical manifestations. Problem-Based Learning (PBL) has been proposed as a strategy to promote this integrative form of reasoning, yet evidence in preclinical physiology, particularly among first-year medical students without prior formal clinical exposure, remains limited. This quasi-experimental pre-post study evaluated the impact of a two-session PBL intervention on physiological reasoning in 135 first-year medical students at the University of Málaga. The intervention comprised six contact hours delivered across five small-group classes (25-30 students per class) organized into teams of five students and was triggered by two real cardiovascular events involving elite athletes. Written responses to two open-ended questions administered before (PRE) and after (POST) the intervention were coded using an ordinal framework (N0-N4) that independently assessed mechanistic conceptual complexity (P1: physiological model of the heart as an effective pump) and physiological cause-effect integration (P2: physiological rationale underlying healthcare intervention following loss of effective blood flow). Both dimensions improved significantly after the intervention. The effect on physiological cause-effect integration was large (P2: Z = -7.723, p < 0.001, r = 0.68), with the proportion of students reaching advanced integrative reasoning levels (N3-N4) increasing from 3.8% to 53.8%. The effect on mechanistic conceptual complexity was moderate (P1: Z = -4.683, p < 0.001, r = 0.41). Improvements in the two dimensions were independent of one another (rs = 0.128, p > 0.05), indicating that they represent distinct cognitive constructs. These findings suggest that brief, carefully designed PBL sequences anchored in authentic clinical problems can produce substantial gains in causal physiological reasoning among first-year medical students, even before the formal introduction of cardiovascular physiology in lecture-based teaching.