Il pseudonimo vs. lo pseudonimo?

I am taking a beginners italian class and we were asked to write the articolo determinativo for a bunch of words. I looked the above word in the dictionary and saw it was a masculin noun so the article could either be "il" or "lo" (since it clearly doesn't begin with a vowel. In my textbook... show more I am taking a beginners italian class and we were asked to write the articolo determinativo for a bunch of words. I looked the above word in the dictionary and saw it was a masculin noun so the article could either be "il" or "lo" (since it clearly doesn't begin with a vowel. In my textbook it says that "lo" is only used with a noun that begins with s + t or "z". Since this one does not begin with either of those, the article has to be "il". My teacher, however said that it was "lo".... Can someone please explain why?

Thanks
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