We present experimental evidence that the effective medium approximation (EMA), [D. C. Morse, Phys. Rev. E 63, 031502 (2001)], provides the correct scaling law of the plateau modulus G0 proportional variantrho4/3Lp(-1/3) (with rho the contour length per unit volume and Lp the persistence length) of semiflexible polymer solutions, in the highly entangled regime. Competing theories, including a binary collision approximation (BCA), instead predict G0 proportional, variantrho7/5Lp(-1/5). We have tested both predictions using F-actin solutions which permit experimental control of Lp independently of other parameters. A combination of video particle tracking microrheology and dynamic light scattering yields independent measurements of G0 and Lp, respectively. Thus we can distinguish between the two proposed laws, in contrast to previous experimental studies focused on the (less discriminating) concentration dependence.