Plane wave scattering by an irregularity slab embedded in a linearly stratified isotropic medium is studied under the single scatter approximation. In addition to regular reflection from the stratified background, both the incident and the reflected waves are scattered by the random irregularities. The behavior of the unperturbed field in the neighborhood of the turning point is accurately taken into account by using the full wave solution. The scattered fields can be interpreted in terms of the Bragg condition similar to the case of the well-known Booker-Gordon form, but with four terms accounting for the reflection effect of the turning point on both the unperturbed and scattered waves. The case of elongated irregularities is studied in detail. Analytical expression for the angular spectrum of the average scattered field intensity is derived, and its physical meaning discussed. In comparison with the case where the background medium is homogeneous, the relation to the two-dimensional Booker-Gordon formula is clarified. Applications to high-frequency propagation in the disturbed ionosphere will be discussed.