An analytic technique is proposed for solving Markov's parabolic equation for the second-order space-frequency coherence function. This technique makes it possible to construct an asymptotic solution for an arbitrary analytic model of the structure function of medium fluctuations. The technique involves the Fourier transformation followed by the use of a quasi-classical method of solution of the corresponding equation for the Fourier transform of an unknown function. Within the scope of the developed approach, all known solutions for the space-frequency coherence function of a field in a medium with fluctuations are obtained as partial solutions.