Treasury Regulations (26 C.F.R.)
26 CFR § 31.3121(b)(13)-1 — (b)(13)-1 Services of student nurse or hospital intern.
(a) Services performed as a student nurse in the employ of a hospital or a nurses' training school are excepted from employment, if the student nurse is enrolled and regularly attending classes in a nurses' training school and such nurses' training school is chartered or approved pursuant to State law.
(b) Services performed before 1966 as an intern (as distinguished from a resident doctor), in the employ of a hospital are excepted from employment, if the intern has completed a 4 years' course in a medical school chartered or approved pursuant to State law.
Source: official text