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I disagree with this definition. It isn't the most obvious (q.v. "walks like a duck"), the most likely (q.v. hoof beats, "common things are common"), nor the most elegant/parsimonious (q.v. Ockham's razor).
It is the highest yield, both in terms of veracity and relevance.
It is why we do ECGs, hCGs, test for fecal occult blood and sometimes limited labs (bedside glucose/hct) on nearly every adult who comes in with syncope. The most common causes (orthostatic, vasovagal) are not going to be identified by this evaluation, and we are looking several reasonably likely/common life-threatening disorders (e.g. ruptured ectopic pregnancy, massive GUI bleed, cardiac dysrhythmia, AMI, hypoglycemia, ruptured AAA, SAH) because that's where the money is.
Also consider the complaint (or worse, solicited response to someone asking if it is) "worst headache of their life". Every SAH I have seen where the person could still speak was having a wicked-bad headache.
Of course, nearly everyone who walks in to the ED with this complaint has a migraine/cluster type headache or the garden variety viral meningitis (the self limiting echoviral type--not the herpesvirus, equine encephalitis, or rabies type).
Sutton's law says we need an LP (usually after CT, maybe even repeating LP in a while if <6 hours of symptoms)--rules out the bad things (bacterial meningitis, SAH, pressure problems, and more serious viral/aseptic meningitis).
Since most of these people (with either benign or odious etiology) also have significant nausea/vomiting, and we know that usual therapy for migraines makes some people with SAH feel better (and I have also seen marked symptomatic improvement in viral meningitis as well), we also give them a dose of a phenothiazine or butyrophenone at the same time, since that is where the money is.
So, it also isn't just diagnostic, as the same approach applies to therapeutic decision making (also think about patients with CHF, HTN, DMII, ASCAD, ESRD, COPD and pulmonary hypertension--these all too common--where you invoke Willie's wisdom with every episode of severe dyspnea/hypoxia).
Cheers, DrK — Preceding unsigned comment added by 75.73.1.89 (talk) 16:46, 4 April 2013 (UTC)