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An old medical proverb is, "When you hear hoofbeats in the night, think of horses, not zebras.1" This is a restatement of the observation that common conditions are common, and rare conditions are rare. We look for the common things first.
A zebra is thus a rare condition which we do not look for first, but we have to include in our list of diagnositic possibilities.
See also Sutton's Law.
1But of course, my cardiology professor from South Africa had it the other way around.