Final answer:
The anatomical guide lateral to the tendon of the flexor carpi ulnaris muscle refers to the U-shaped grouping of carpal bones and the flexor retinaculum, which together form the carpal tunnel allowing passage of tendons and nerves.
Step-by-step explanation:
The anatomical guide that lies lateral to the tendon of the flexor carpi ulnaris muscle refers to the U-shaped grouping of the carpal bones in conjunction with the flexor retinaculum. This structure is significant in forming the carpal tunnel, which serves as a passageway for nine muscle tendons and an important nerve entering the hand from the anterior forearm. The extensor carpi ulnaris, which extends and adducts the hand, is located on the posterior side of the forearm and is not associated directly with the flexor carpi ulnaris but rather with the superficial posterior compartment of the forearm alongside other muscles such as the extensor digitorum and extensor digiti minimi.