To answer this question, we need to find out how many mg of diphenhydramine are in  one dose of 15 mL, and then multiply by the number of doses in a day. The bottle is labeled, diphenhydramine 12.5 mg/5 mL, which means that every 5 mL of  liquid contains 12.5 mg of diphenhydramine. To find out how many mg of  diphenhydramine are in one dose of 15 mL, we can use a proportion: 

12.5 mg / 5 mL = x mg / 15 mL 

Cross-multiplying and solving for x, we get: 

x = (12.5 x 15) / 5 

x = 37.5 

Therefore, one dose of 15 mL contains 37.5 mg of diphenhydramine. 

The prescription is for every 12 hours PRN, which means that the client can take up to  two doses in a day as needed. The total daily dose in mg that the client receives is  therefore: 

37.5 x 2 = 75 mg