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transfused for an elderly woman with anemia. Between each task, my thoughts returned to what had happened with Mr. Morgan. In medical school physiology lectures I learned the relevant formulas for cardiac output and gas exchange in the lungs; in pharmacology class, the actions of various medications on heart muscle. On bedside rounds, I had spent hours listening to the sounds of patients' hearts. But I had no idea what I was hearing in Mr. Morgan's chest, or what to do about it. My high grades were meaningless. The MGH selection committee had made a mistake offering me an internship. After all the years of preparation, I ended up with an empty head and my feet fixed to the floor.
    Mercifully, the rest of the night was uneventful. Three patients were admitted, but none was very ill, and most of their evaluation had been completed in the ER before they were transferred upstairs to the Baker service. Around 3 A.M., I called the OR. I heard that Mr. Morgan had survived open heart surgery, a prosthetic valve firmly in place. My shoulders slumped in relief.
     
     
    That first night of internship showed me that I needed to think differently from how I had learned to think in medical school—indeed, differently from the way I had ever thought seriously in my life. This was despite my having met patients like Mr. Morgan before. During medical school we had studied what are called paper cases, patients in the form of written data. The attending physician would hand out a detailed description that would begin something like this: "A 66-year-old African-American retired postal worker with a history of poorly controlled hypertension presented to the hospital with the chief complaint of worsening chest pain over several weeks. Initial evaluation ruled out angina. On the third day of the hospital stay, he developed acute respiratory distress." The attending would then give more details on Mr. Morgan—the range of his elevated blood pressure, the medications that failed to control it in the past—and lead us through a systematic analysis of the problem. First, the chief complaint, here acute shortness of breath. Second, the history of the present illness, angina having been ruled out. Third, the medical history, notably poorly controlled hypertension. Fourth, the physical examination. At that point, the attending would elaborate on what was heard through the stethoscope: breath sounds described as "rales," indicating fluid in the lungs; another heart sound, an "S3," indicating cardiac failure; and the crescendo/decrescendo murmur of aortic regurgitation—blood being pumped out through the left ventricle into the aorta but then flowing back into the heart.
    Hands would shoot up in the classroom as students offered their ideas about what was wrong. Our mentor would take these hypotheses and write them on the board, creating a "differential diagnosis," a laundry list of possible causes of sudden shortness of breath in a man with this medical history and these physical findings. From this differential diagnosis, he would point to the right answer and then enumerate the measures taken to restore respiratory and cardiac function until the patient was placed on heart-lung bypass in the OR.
    In the last two years of medical school, when we saw patients on bedside rounds, the attending physician modeled a similar intellectual strategy for us. He would lead us through a calm, deliberate, and linear analysis of the clinical information and how to treat the malady.
    As Robert Hamm of the Institute of Cognitive Science at the University of Colorado, Boulder, contends, the irony is that our mentor, the senior attending physician, does not think this way when he actually encounters a patient like William Morgan. At such moments, Hamm writes, it is not evident that any "reasoning" is being used at all. Studies show that while it usually takes twenty to thirty minutes in a didactic exercise for the senior doctor and students to arrive at a working