Conditions for a z test about a proportion

Moussa saw a commercial on television that claimed $9$9 out of $10$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$50 dentists and asked whether or not they recommend using that brand of chewing gum. He wants to use the sample data to test $H_0:p=0.9$H, start subscript, 0, end subscript, colon, p, equals, 0, point, 9 versus $H_\text{a}:p<0.9$H, start subscript, start text, a, end text, end subscript, colon, p, is less than, 0, point, 9, where $p$p is the proportion of all dentists that recommend this brand of chewing gum.