  A recent proposal to experimentally test quantum mechanics against
noncontextual hidden-variable theories [Phys. Rev. Lett. 80, 1797 (1998)] is
shown to be related with the smallest proof of the Kochen-Specker theorem
currently known [Phys. Lett. A 212, 183 (1996)]. This proof contains eighteen
yes-no questions about a four-dimensional physical system, combined in nine
mutually incompatible tests. When these tests are considered as tests about a
two-part two-state system, then quantum mechanics and non-contextual hidden
variables make the same predictions for eight of them, but make different
predictions for the ninth. Therefore, this ninth test would allow us to
discriminate between quantum mechanics and noncontextual hidden-variable
theories in a (gedanken) single run experiment.
