Final answer:
The musculocutaneous nerve is the nerve most frequently missed in an interscalene brachial plexus block. For forearm extension, the triceps brachii is the responsible muscle, with other muscles serving different roles in arm movement.
Step-by-step explanation:
In an interscalene brachial plexus block (ICB), the nerve that is most often missed due to its location and innervation patterns is the musculocutaneous nerve. It is not uncommon for anesthetists to miss this nerve because it often separates from the brachial plexus early and travels through the coracobrachialis muscle, which can make it harder to anesthetize effectively compared to the other nerves of the brachial plexus like the median, ulnar, and radial nerves.
Forearm Muscle Function
As for the question regarding which muscle extends the forearm, the correct answer is b) triceps brachii. This muscle is responsible for the extension of the elbow joint, counteracting the flexion action of the biceps brachii. The biceps brachii and brachialis are mainly involved in the flexing of the forearm, while the deltoid is primarily a shoulder muscle responsible for arm abduction.