Q. A 25-year-old male marathon runner comes in for a routine physical examination, he reports he has been training for a marathon for the past month. He states that he has been drinking water and electrolyte replacement drinks during his runs. On examination, he is found to be in good health and has no abnormalities. His vital signs are within normal limits. His blood pressure is 120/80 mm Hg, heart rate is 72/min, and he has no signs of edema or jaundice. A urinalysis is performed and demonstrates the presence of hyaline casts. The urine specific gravity is 1.010, which is considered normal. The patient reports that he has been experiencing muscle cramps and fatigue during his runs. Which of the following is the most likely diagnosis?