A client 2 days post abdominal surgery develops a fever of 101.8°F (38.8°C). What is the most common cause of post-operative fever within the first 24-48 hours?

POST-OPERATIVE FEVER — TIMING-BASED APPROACH: DAY 1 (within 24 hours): ATELECTASIS (collapsed lung segments) is the most common cause; wound infections at this stage are nearly impossible (too early for bacteria to proliferate); TREATMENT: Incentive spirometry (10 deep breaths every hour); early ambulation; pain management to allow deep breathing; DAY 3-5: WIND (pneumonia), WATER (urinary tract infection from catheter), WOUND (surgical site infection beginning to develop), WALKING (DVT/PE); DAY 5-7: WOUND infection peaks; IV site infection (phlebitis, catheter infection); DEEP VEIN THROMBOSIS fever; LATE (weeks): Internal abscess, deep wound infection; INCENTIVE SPIROMETRY RATIONALE: Surgery + anesthesia → splinting (shallow breathing from pain) + reduced respiratory drive → alveolar collapse → microatelectasis → fever as inflammatory response + bacterial colonisation of collapsed areas → pneumonia if untreated. Early ambulation and breathing exercises prevent this cascade.