A 52-year-old woman has a 2-week history of watery diarrhea without blood. The patient states that 4 to 5 weeks ago she enjoyed backpacking vacation and drank water from mountain streams. She was sure the water was potable, as the unspoiled, pristine area abounded with fish, beaver, and plant life. She states she has enjoyed perfect health except that she takes antacids for what she describes as gastrosophageal reflux disease. Her physical examination produced unremarkable findings. Examination of liquid stool revealed trophozoites and cysts of Giardia lamblia. Which of the following is the correct treatment for this disease? A. Mefloquine B. Mebendazole C. Meglumine antimonate D. Metronidazole