A 23-year-old patient in the 27th week of pregnancy has been hospitalized on complete bed rest for 6 days. She experiences sudden shortness of breath, accompanied by chest pain. Which of the following conditions is the most likely cause of her symptoms?

In a hospitalized patient on prolonged bed rest, the most likely cause of sudden onset shortness of breath and chest pain is pulmonary embolism. Pregnancy and prolonged inactivity both increase the risk of clot formation in the deep veins of the legs, known as deep vein thrombosis (DVT). These clots can dislodge and travel to the lungs, causing a pulmonary embolism. Myocardial infarction (Choice A) is less likely in a young patient without a significant history of atherosclerosis. Anxiety attacks (Choice C) may present with similar symptoms but are less likely in this context. Congestive heart failure (Choice D) is less probable given the acute onset of symptoms and absence of typical signs like peripheral edema in this case.