Final answer:
The most likely reason for the teenager's symptoms, given their diagnosis of Marfan's Syndrome and the observed diminished lung sounds, is spontaneous pneumothorax.
Step-by-step explanation:
The reason a 17 year old teenager with Marfan's Syndrome is suddenly short of breath and has diminished lung sounds on the left side of the chest and dyspnea is most likely due to spontaneous pneumothorax. Marfan's Syndrome is associated with connective tissue abnormalities, which can affect the lungs and lead to a spontaneous pneumothorax, or collapsed lung. The diminished lung sounds on one side of the chest are indicative of this condition, where air leaks into the space between the lung and chest wall, leading to lung collapse. Unlike pneumonia, which is often accompanied by a productive cough and crackling sounds, or a pulmonary embolism that typically presents with a rapid onset of shortness of breath and chest pain, the patient's presentation is more consistent with the symptoms of a pneumothorax.