This interrupted case study in renal physiology and water balance follows the story of "Lacy," a star athlete, who makes poor fluid intake choices for the duration of a track and field day. Lacy notices changes in the volume and appearance of her urine as the day progresses, and eventually succumbs to suspected dehydration. Students work through questions about general body water inputs and outputs, the structure of the nephron and the locations within the nephron where water and sodium ion reabsorption take place. The hormonal control of water and sodium ion reabsorption by vasopressin (antidiuretic hormone) and aldosterone are also deliberated. Students then consider how nephron function is affected by the consumption of caffeine and the implications of body water imbalances on the functioning of the cardiovascular system. This case was developed for use in an upper-level human physiology course taken over two semesters and could be adapted for a lower-level physiology problem-based learning tutorial or extended for a pathophysiology course.