A 30-year-old patient is injured in a motor vehicle collision.

The diaphragm is innervated by the phrenic nerve, which originates from spinal nerve roots C3–C5. NREMT teaching highlights the phrase: “C3, 4, and 5 keep the diaphragm alive.”

Option A (C4) is correct because an injury at or above this level can paralyze the diaphragm, resulting in apnea and immediate respiratory failure.

Option B (T1) and C (T3) affect intercostal muscles but not the diaphragm.

Option D (L4) affects lower extremity function only.

NREMT emphasizes that high cervical spine injuries are immediately life-threatening due to loss of ventilatory control.