Final answer:
The most likely etiological agent is enterotoxigenic Escherichia coli (ETEC), a bacterial infection.
Step-by-step explanation:
The most likely etiological agent for a patient presenting with vomiting and profuse, watery diarrhea 3.5 hours after eating chicken fried rice is likely to be a bacterial infection, specifically caused by enterotoxigenic Escherichia coli (ETEC).
ETEC is a common cause of traveler's diarrhea, and it is transmitted through contaminated food and water. Symptoms usually begin within a few days of ingestion and can include watery diarrhea, vomiting, and abdominal cramping.
A blood test may not directly reveal the specific etiological agent, but it can indicate the presence of an infection by showing an elevated white blood cell count or increased levels of C-reactive protein.