Which of the following letters is NOT a valid starting letter for a prescriber's DEA number?

In the context of prescriber DEA numbers, the valid starting letters are significant because they indicate the type of prescriber. The letters A, B, and M are recognized as valid starting letters for DEA numbers associated with different categories of practitioners. For example, letter A typically denotes a physician, while letter B is used for practitioners who are applying for registration after the enactment of the Controlled Substances Act.

However, the letter K is not a valid starting letter for a prescriber's DEA number. The letters A, B, D, and M cover a range of practitioners such as physicians, dentists, and nurse practitioners, but K is generally reserved for non-practitioner entities, such as hospitals or institutions. Thus, option B being the answer to the question reflects this fact, positioning it as the only option that does not correspond to a legitimate starting letter for a DEA number assigned to individual prescribers.