  We study $pK^-$ scattering in the energy range from threshold through the
$\Lambda(1520)$ peak in UBChPT, taking into account $\mathcal{O}(q)$ vertices
from meson-baryon contact interactions and $s$- and $u$-channel ground-state
baryon exchange, $s$- and $u$-channel decuplet- and nonet-baryon exchange and
$t$-channel vector-meson exchange, as well as $\mathcal{O}(q^2)$
flavor-breaking vertices. Detailed fits to data are presented, including a
substantial body of differential cross-section data with meson momentum
$q_\mathrm{lab} > 300$ MeV not considered in previous treatments.
