Conditions for a z test about a proportion

Moussa saw a commercial on television that claimed $9$‍ out of $10$‍ dentists recommend using a specific brand of chewing gum. He suspected that the true proportion was actually lower, so he took an of $50$‍ dentists and asked whether or not they recommend using that brand of chewing gum. He wants to use the sample data to test $H_0:p=0.9$‍ versus $H_{\text{a}}:p<0.9$‍, where $p$‍ is the proportion of all dentists that recommend this brand of chewing gum.