  We consider light scattering by an anisotropic defect layer embedded into
anisotropic photonic crystal in the spectral vicinity of an optical bound state
in the continuum (BIC). Using a resonant state expansion method we derive an
analytic solution for reflection and transmission amplitudes. The analytic
solution is constructed via a perturbative approach with the BIC as the zeroth
order approximation. The solution is found to describe the collapsing Fano
feature in the spectral vicinity of the BIC. The findings are confirmed via
comparison against direct numerical simulations with the Berreman transfer
matrix method.
