A 9-month-old child is brought to the Health Depament to receive the second dose of oral polio vaccine, 2 weeks after the first vaccination. The child has mild diarrhea, so the decision is made to defer fuher immunizations. Bacteriologic examination of a stool culture is unremarkable; however, a small, single-stranded, positive RNA virus is isolated from the specimen. The viral isolate was not inactivated by ether. Which of the following viruses was most likely isolated?