We studied gains in student learning over eight semesters (2002-2011) in which an introductory biology course curriculum was changed to include optional verbal final exams. Students could opt to demonstrate their mastery of course material via a structured oral exam/clinical interview with the professor. In a quantitative assessment of cell biology content knowledge, students who passed the verbal final (VF) exam outscored their peers on the MAT (66.4% n=160, 62% n=285, respectively; p<0.001); an exam built with MCAT questions. The higher achieving students performed better on MCAT questions in all topic categories tested; the greatest gain occurred on the topic of cellular respiration. Since the verbal final exam focused on a conceptually parallel topic, photosynthesis, there may have been authentic knowledge transfer. Participation and success passing the verbal final exam had balanced representation in terms of gender, ethnicity and academic standing. In longitudinal studies, passing also correlated with higher performance in a range of upper-level science courses, with greatest significance in biochemistry and organic chemisty. Students who attempted, but did not pass, the VF achieved higher scores on the MAT instrument as well as on the course's written final exam compared to peers. Students self-report increased time on task, deeper study, discovering interconnections and valuing verbalization. Whether they participated or not, students nearly unanimously (92% "Much or Great Help," n=243) strongly valued the option. The evidence evaluated in this study supports that Socratic-styled probing oral exams at the introductory level can allow instructors to assess and aid students striving to achieve higher-level learning.