Here are the actual laws taken from the citations in the document above. These laws were written in 1994, long before there were DNP, thus the differentiation between CRNAs and MDs.
§213. Exemptions
(a) Minimum wage and maximum hour requirements
The provisions of sections 206 (except subsection (d) in the case of paragraph (1) of this subsection) and section 207 of this title shall not apply with respect to-
(1) any employee employed in a bona fide executive, administrative, or professional capacity (including any employee employed in the capacity of academic administrative personnel or teacher in elementary or secondary schools), or in the capacity of outside salesman (as such terms are defined and delimited from time to time by regulations of the Secretary, subject to the provisions of subchapter II of chapter 5 of title 5, except that an employee of a retail or service establishment shall not be excluded from the definition of employee employed in a bona fide executive or administrative capacity because of the number of hours in his workweek which he devotes to activities not directly or closely related to the performance of executive or administrative activities, if less than 40 per centum of his hours worked in the workweek are devoted to such activities)
§ 541.304 Practice of law or medicine.
(a) The term “employee employed in a bona fide professional capacity” in section 13(a)(1) of the
Act also shall mean:
(1) Any employee who is the holder of a valid license or certificate permitting the practice of law or medicine or any of their branches and is actually engaged in the practice thereof; and
(2) Any employee who is the holder of the requisite academic degree for the general practice of medicine and is engaged in an internship or resident program pursuant to the practice of the profession.
(b) In the case of medicine, the exemption applies to physicians and other practitioners licensed and practicing in the field of medical science and healing or any of the medical specialties practiced by physicians or practitioners. The term “physicians” includes medical doctors including general practitioners and specialists, osteopathic physicians (doctors of osteopathy), podiatrists, dentists (doctors of dental medicine), and optometrists (doctors of optometry or bachelors of science in optometry).
(c) Employees engaged in internship or resident programs, whether or not licensed to practice prior to commencement of the program, qualify as exempt professionals if they enter such internship or resident programs after the earning of the appropriate degree required for the general practice of their profession.
(d) The requirements of
§ 541.300 and subpart G (salary requirements) of this part do not apply to the employees described in this section.