All participants will undergo detailed clinical evaluation, including history, examination, routine and relevant investigations, and pre-anaesthetic assessment on the day prior to surgery. Patients will be briefed about the Numeric Rating Scale (0–10) for pain assessment and randomly allocated into two groups using block randomization: Group C (Conventional anterior quadratus lumborum block) and Group S (Subcostal anterior quadratus lumborum block). Written informed consent will be obtained.

Following standard intraoperative monitoring and spinal anaesthesia (17.5 mg of 0.5 percentage hyperbaric Bupivacaine at L3–L4), patients will undergo percutaneous nephrolithotomy in the prone position. At the end of surgery, Group C will receive a conventional anterior quadratus lumborum block and Group S will receive a subcostal approach, both under ultrasound guidance using 20 ml of 0.25 percentage Bupivacaine. The block technique and time will be recorded.

Postoperative pain will be assessed using NRS at rest (2, 4, 8, 12, 24 hours) and during movement (12, 24 hours). Rescue analgesia (IV Tramadol 0.5 mg/kg) will be administered if NRS greater than or equal to 4, with total 24-hour consumption noted. Hemodynamic parameters and side effects such as nausea, vomiting, site pain, hematoma, or local anaesthetic toxicity will be monitored. All outcome assessments will be done by a blinded observer.