What is this long thin mark on my lens, fungus or dust?
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Unfortunately, the lighting and focus makes it difficult to tell. But just based on the upload photo, it looks like either a fine piece of lint or dust stuck to the inside of the rear element.
As Tetsujin comments, the mark looks like a scratch, but the photos are not clear, so it could be a long piece of lint, as scottbb suggests. It is unlikely to be dust, which are usually small roundish particulates, or fungus, which usually have a branching structure.
If the mark is a piece of lint on the outside of the lens, you can try using a blower to remove it. If it is on the inside and it really bothers you, the inside of the rear element is easily accessible on many lenses. However, I recommend leaving it alone because the of the likelihood of making things worse while trying to remove it.