When culturing C. diphtheriae, which of the following media will enhance pleomorphism and granule production?

The correct choice for enhancing pleomorphism and granule production in C. diphtheriae is Loeffler serum agar. This medium is particularly designed to support the growth of Corynebacterium species by providing the necessary nutrients and conducive environment that promote their characteristic features.

Corynebacterium diphtheriae is known for its ability to appear in various shapes and arrangements, known as pleomorphism, and to produce metachromatic granules. Loeffler serum agar, which is rich in serum and nutrients, encourages optimal growth conditions for this organism, thus enhancing both its pleomorphic characteristics and the production of these granules.

Other media listed serve different purposes: Nutrient agar provides a general growth medium but lacks specific components that would foster the distinctive features of C. diphtheriae. MacConkey agar is selective for gram-negative bacteria, inhibiting the growth of gram-positive organisms, including C. diphtheriae. Blood agar also supports a variety of organisms but does not specifically enhance the pleomorphism or granule production as effectively as Loeffler serum agar does.