Which might be true, of course, in certain circumstances - if you were in practice in the Serengeti, for example (curiously, I was once in the Serengeti, heard hoof-beats outside my window, peered through the early morning mist and saw only an old cow).
Because common things are common and uncommon presentations of common diseases are more common than common presentations of uncommon diseases. But sometimes ...
As an intern, I saw a young lad in casualty. He had fainted at a disco (yes, it was that long ago, Saturday Night Fever was quite fashionable. Old age is creeping up on me, not sure why but fairly sure it's up to no good) and he had a few unusual skin lesions and a labile BP.
These days, I doubt if I would be able to recognise a phaeochromocytoma if one walked up and assaulted me with a blunt speculum (I've been flogged into apathy by too many URTIs and sick certs, rare and interesting diseases only present to other doctors), but I was young then, fresh and sharp and so hip, I could hardly see over my pelvis.
I wrote 'possible neurofibromatosis?', 'possible phaeo?' on the chart and admitted the young man to the ward. I was too green to realise the importance of hoarding unusual cases to myself, for my own advancement, and sure enough, the rumour spread around the hospital as fast as an epidemic of flaming gonorrhoea.
Later, when I went to check up on my patient, I found him buried under a tide of medical students, SHOs and research registrars, all keen for a piece of the glory, all ordering 24-hour urines, all dreaming of a case report for the peer-reviewed journals and another notch on their CVs.
'Help me, doc,' he said, desperately, 'they're suffocating me.' I whipped away the students, but the others were far above me in the hierarchy and I could offer little succour. 'Sorry, pal,' I said. 'It's a common complication of uncommon diseases.'
- Dr Farrell is a GP from County Armagh. Follow him on Twitter @drlfarrell.