"Lui non risponde più ad alcuna domanda."

[JamesAdam]

Is the singular use of 'domanda' correct here? Should it not be 'domande'?

[f.formica]

Yes; alcuna here means "not even one" and could just as well be "nessuna". Both are considered singular; if you were to use "alcune domande" making it plural, the meaning would change in "some questions", i.e. he won't answer some questions but might answer others.

[Hornstein]

Not even one question - domanda - is singular, so why give the plural as correct