The guidelines for differential diagnosis are... sometimes embarrassing to look at. But I'm not sure they could be better.
What do you do when lupus erythematosus can present with identical symptoms to an autoeczematous id reaction to a fungal tinea incognito infection on your damn shin? The doctor throws you on steroids... Why? Because if you really do have lupus, at least this stands a good chance of prolonging your life. Unfortunately, if you don't, it can shorten it. But you may never find that goddamn asymptomatic fungal infection.
It also doesn't help that doctors aren't biochemists. Not by a long shot. They're car mechanics, but they don't really understand VE curves or stoichiometric ratios. On average, they're shit without a Chilton manual.