In physiology education, students must learn to recognize and construct causal explanations. This challenges students, in part, because causal explanations in biology manifest in different varieties. Unlike other natural sciences, causal mechanisms in physiology support physiological functions and reflect biological adaptations. Therefore, students must distinguish between questions that prompt a proximate or an ultimate explanation. In the present investigation, we aimed to determine how these different varieties of student knowledge coordinate within students' written explanations. Prior research in science education demonstrates that students present specific challenges when distinguishing between proximate and ultimate explanations: students appear to conflate the two or construct other nonmechanistic explanations. This investigation, however, demonstrates that analytic frameworks can distinguish between students' proximate and ultimate explanations when they are provided explanatory scaffolds that contextualize questions. Moreover, these scaffolds and prompts help students distinguish between physiological functions and the cellular and molecular mechanisms that underpin them. Together, these findings deliver insight into the context-sensitive nature of student knowledge in physiology education and offer an analytic framework for identifying and characterizing student knowledge in physiology.