This variability has led many people to think of it as a spectrum of symptoms rather than a single, clear-cut syndrome. And that variability makes it hard to work out what causes it.

这种多变性令很多人认为它的症状多样而不是单一明确的，这种多样化加大了对其病因的研究难度。

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This variability has led many people to think of it as a spectrum of symptoms rather than a single, clear-cut syndrome. And that variability makes it hard to work out what causes it.

这种多变性令很多人认为它的症状多样而不是单一明确的，这种多样化加大了对其病因的研究难度。

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