Which muscle is innervated by the oculomotor nerve (CN III)?

The pupillary muscle, specifically the sphincter pupillae, is innervated by the oculomotor nerve (CN III). This muscle is responsible for constricting the pupil in response to light and during accommodation for near vision. The oculomotor nerve provides the parasympathetic innervation necessary to control these actions, which is vital for regulating the amount of light entering the eye and ensuring proper visual focusing.

Analyzing the other options, the cardiac muscle is innervated by the autonomic nervous system rather than a single cranial nerve. The rectus abdominis, a muscle of the abdomen, is primarily innervated by the lower thoracic nerves. The diaphragm, which is crucial for respiration, is mainly innervated by the phrenic nerve, originating from the cervical plexus. Understanding the specific roles and innervations of these muscles highlights why the pupillary muscle is the correct answer related to the oculomotor nerve.