Q. A 35-year-old man presents to the emergency department with a sudden onset of severe, sharp, unilateral pain in the left flank that radiates to his testes. He also reports nausea, vomiting and a history of recurrent kidney stones. He has been experiencing these symptoms for the past 6 hours and rates the pain as an 8 out of 10 on a visual analog scale. He reports that he has had similar episodes in the past and that they usually resolve after passing a kidney stone. Physical examination reveals costovertebral angle tenderness, and urinalysis shows the presence of envelope-shaped crystals. The patient's blood pressure is 130/80 mm Hg, heart rate is 90 beats per minute and temperature is 37.5 degrees Celsius. The patient is otherwise healthy and does not have any known medical conditions. Based on the urinalysis and patient’s history, which type of kidney stone is most likely causing his symptoms?