  We report a quantum magnetotransport signature of a change in Fermi surface
topology in the Rashba semiconductor BiTeI with systematic tuning of the Fermi
level $E_F$. Beyond the quantum limit, we observe a marked increase/decrease in
electrical resistivity when $E_F$ is above/below the Dirac node that we show
originates from the Fermi surface topology. This effect represents a
measurement of the electron distribution on the low-index ($n=0,-1$) Landau
levels and is uniquely enabled by the finite bulk $k_z$ dispersion along the
$c$-axis and strong Rashba spin-orbit coupling strength of the system. The
Dirac node is independently identified by Shubnikov-de Haas oscillations as a
vanishing Fermi surface cross section at $k_z=0$. Additionally we find that the
violation of Kohler's rule allows a distinct insight into the temperature
evolution of the observed quantum magnetoresistance effects.
