A 12 years old boy presents with a 1-week history of sudden-onset watery diarrhea while visiting relatives in Dakar, Bangladesh. On his return to the US, he developed abdominal cramps, fever, and bloody diarrhea. He is febrile, with a temperature of 101.2°F (38.5°C). The mucous membranes are dry, and there is reduced skin turgor; his pulse is 100 beats/ minute and BP is 110/70 mmHg with a 10 mmHg postural drop. What is the most likely diagnosis?