The doctor will advise you what is the best treatment.

The phrase "what is" should be revised for clarity and grammatical accuracy in this context. The correct construction involves introducing a noun clause that serves as the direct object of the verb "advise." This typically would translate to "what the best treatment is," thereby presenting the information in a grammatically coherent manner.

In this context, "what is" fails to connect properly with the rest of the sentence, creating confusion. The doctor is not simply telling the recipient about the existence of the best treatment but rather providing specific guidance regarding it. By rephrasing to include "what the best treatment is," the structure fulfills the requirements of standard English grammar and enhances clarity.

This restructuring emphasizes the importance of noun clauses following verbs that convey advice or information, ensuring that the sentence communicates clearly what is being advised.