A 17-year-old high school football player presented to a neurology clinic because his mother thought that he may have acquired neck problems during a game. A month before, he had sustained a concussion from a blow to his head from another player. Shortly after, she noted that he intermittently tilted his head to the side. When asked what was the matter, he simply said that sometimes he had double vision, and that the images were situated on top of each other vertically, making it difficult to go down stairs. When examined, there was no neck pain or limitation of motion. He tended to keep his head tilted to the right side. When asked to follow the doctors finger with his head in a straight position, his left eye would not move downward when his eyes were turned to the right, and tended to remain slightly deviated toward the left. At this point, he stated that he had double vision, and felt better if his head was tilted to the right. The remainder of his eye movements, as well as the remainder of his exam, was normal. Which of the following muscles is weakened?