Kitty crystals in urine

My 5 month old kitten has crystals in her urine. I've had her since 14 weeks or so and had been feeding her blue buffalo grain free soft food so I'm not sure why she has crystals. Within two days of bringing her home she got an upper respiratory infection which she's now been fighting off for a 1.5 months. I don't know if that has anything to do with her crystals - maybe because her immune system is low? 

The vet put her on Hills Prescription Diet C/D for urinary problems. She eats mostly the soft food for but I also have the hard for the older cat (no problems but it's easier than trying to keep the little one from eating regular food). 

I don't know if I'm thinking too much, but I've read that Hills is not the best to feed our little bugs, that's why I was feeding the grain free blue buffalo. My vet thinks I'm crazy and says that the prescription diet food is good for them. 

Is there another kind/brand of food that you would recommend for crystals? If she has crystals will she always have to eat the prescription food? BTW she is on lysine for the URI.

Any advice is helpful. Thank you.

Welcome to the Cat Site! The crystal problem is not due to the food, but to lack of water. Wet food has more water in it to begin with, and you can add more water to it to make a slurry. Take her off the dry food for a while until the crystals clear up, then you can add a little to her feeding schedule. But for the most part, continue feeding her wet food mixed with water.

Thank you! I'll add water to the soft food.

My cat Hunter has been to the vet twice for this. Actually, he never actually formed crystals, but he did have trouble urinating and the vet suggested we switch to a primarily wet, low carb diet to help him get more fluids. it seems to have helped.