For a prescription of RX -3.50 -1.75 X 180 and K's of 42.00 @ 180 and 42.50 @ 90, what lens design would provide the best visual result?

The best visual result for a prescription of RX -3.50 -1.75 X 180, along with the specified K values, is achieved with a soft toric lens. This is due to the fact that the patient has a significant astigmatic component in their prescription (-1.75 at axis 180), which requires a lens that can correct for this astigmatism effectively.

Soft toric lenses are specifically designed to account for both the spherical and cylindrical aspects of a prescription. They provide a stable platform that aligns with the orientation of the astigmatism, ensuring clearer vision by compensating for the irregular shape of the cornea. The toric design allows the lens to sit correctly on the eye, maintaining its position during blinking and eye movement, which is crucial in astigmatic corrections.

In contrast, gas permeable spherical lenses may not adequately address the astigmatic portion of the prescription since they primarily correct for spherical errors. Standard contact lenses typically refer to spherical designs as well, which won't effectively manage the cylinder component. Rigid gas permeable lenses may also fail to provide the necessary correction for astigmatism unless specifically tailored as toric designs.

Thus, the soft toric lens is the most appropriate choice