A 5 year old patient is seen at your clinic and complains of "pain here", as he rubs his chest. His mother says that he has had frequent bouts of coughing off and on for 3 weeks. He has not been as energetic as usual, which the mother says has been a relief at times, as he is normally a bit rambunctious. The mother and father are non smokers, but the boy stays with his grandmother while the parents are at work, and the grandmother is a heavy smoker. The child is in the 60th percentile for weight, and 50th percentile for height. Body temperature is 98.6 F, oxygen saturation (SpO2) is 90%. Lung sounds include crackles and wheezing. The heart sounds have strong, regular S1 and S2. Eardrums appear normal, as do the whites of the eyes and retinas. Of the following choices, which is the most probable issue?