derpymdUworld says the most common cause is nontypeable H. flu, S. pneumo next, followed by Moraxella. That said, nontypeable H. flu wasn't an answer+2
tinylilronPer AMBOSS: "S. pneumoniae and H. influenzae account for โผ75% of cases, then M. catarrhalis. Staphylococci and gram-negative organisms predominate in chronic sinusitis."+
submitted by โbwdc(697)
Consider bacterial sinusitis to be analogous to bacterial pneumonia. All are possible, but Strep pneumo is the most common.