A 27-year-old woman presents with a 2-day history of fever, worsening headache, photophobia, vomiting, and increasing confusion. On arrival she is alert but intermittently disorientated.

Vitals: HR 112 bpm, BP 108/64 mmHg, RR 22/min, Temp 38.9°C, SpO₂ 98% on air.

 Exam: Positive Kernig’s sign, mild neck stiffness, no focal neurology, pupils equal and reactive.

 Labs: WCC 17.2 ×10⁹/L, CRP 124 mg/L, platelets normal, clotting normal, glucose 6.4 mmol/L.

Your ST3 registrar assesses her, initiates IV ceftriaxone , contacts the medical team, and prepares for a lumbar puncture under your supervision.

 Before performing the procedure, he asks you to confirm the correct surface landmark that identifies the appropriate vertebral level for safe LP insertion below the termination of the spinal cord.

Which anatomical landmark is used to identify the correct level for lumbar puncture?