It is necessary to rotate the patient's foot 30 degrees medially for a medial oblique foot radiograph to:

Rotating the patient's foot 30 degrees medially for a medial oblique foot radiograph is essential to reduce superimposition of the cuboid bone. This specific positioning allows the radiographic projection to visualize the bones of the foot more clearly, particularly the cuboid, and to minimize overlapping of structures, which can lead to diagnostic errors.

In an oblique view, the desired outcome is to open up the lateral aspect of the foot, making the cuboid more visible while minimizing any overlapping with adjacent bones such as the calcaneus and the fourth and fifth metatarsals. This technique is crucial for accurate assessment of the foot's anatomy and any pathologies that may be present.

Other choices, while important aspects of foot radiography, are not the primary reason for the medial oblique rotation. For instance, focusing on the navicular bone or highlighting the fifth metatarsal may require different positioning or angling techniques, and aligning the distal phalanges is generally not the main goal of this specific view. Each of these aspects may relate to imaging the foot but do not directly pertain to the necessity of the 30-degree medial rotation in reducing superimposition of the cuboid bone.