Q. A 5-year-old unvaccinated boy is admitted to the hospital with severe diarrhea. The patient has been experiencing frequent and watery stools for the past three days, and has also been experiencing nausea and vomiting. The patient's parents report that he has been drinking a lot of fluids, but that he is still dehydrated and has a sunken appearance to his eyes. Physical examination reveals that the patient is slightly dehydrated, with dry mucous membranes and skin turgor. His abdomen is slightly distended and tender to the touch, with mild guarding. The patient's heart rate and blood pressure are elevated, and he is noted to have poor skin perfusion. You order laboratory tests, including a complete blood count, electrolyte panel, and stool analysis. The results show that the patient is dehydrated, with elevated levels of glucose, urea, and creatinine, as well as low levels of potassium and sodium. The stool analysis is positive for a pathogen, as confirmed by an ELISA test and negative-stain transmission electron microscopy (see image). Which of the following is the most likely causative agent?