TIL: The "original antigenic sin" is a phrase describing a fundamental flaw in the immune system's ability to adapt over time. When the immune system is first exposed to an antigen (bad thingy), it tailors the best possible adaptive response to that threat. The problem is that the threat (be it a bacterium, virus, etc) mutates significantly by the time the person is re-exposed to the same antigen. The immune response, though, doesn't change. It's like you finally learn that Achilles' weakness is his heel and you proceed to keep attacking the heel of every descendant of Achilles that you fight.
We learned last year that the point of maximal impulse (PMI) is a physical exam finding of the location where you can best feel the beating of the heart through the chest wall. This location is usually at the counterintuitively named apex of the heart along the midclavicular line in the 5th intercostal space. Today we finally learned that the reason we care about this heart measurement is because different pathological processes affect this location differently. Whereas ventricular hypertrophy only changes the magnitude of the PMI, hypervolemic distention of the ventricles will also displace the PMI to the left.
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