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Never Suicide, despite the name, is actually about "murder is covered up to appear as a suicide". But cover-ups, by nature, are rarely foolproof on closer examination.
I'm asking specifically about the allergy aspect. Never Suicide is already listed for this work.
The allergy is far too specific an aspect to be a trope. It's a clue that leads the detective to the cover-up of murder as "suicide".
The concept of a clue that would rule out suicide seems tropable, basically the counterpart to Disability Alibi.
A woman had apparently comitted suicide. In fact, her wool gloves have gun shot residue. Oops—ahe was extremely allergic to wool. Since her hands show no sign of an allergic reaction, the golves could only have been put on her hands after death.