A post-operative client develops a fever of 101.5°F (38.6°C) on day 2 after abdominal surgery. What is the MOST common cause of early post-operative fever (within the first 48 hours)?

Post-operative fever has a timing-based differential using the '5 W's' mnemonic: WIND (POD 1-2): ATELECTASIS — partial alveolar collapse from splinting (guarding breathing to avoid pain), anesthesia effects, and secretion retention; causes fever due to inflammatory response; TREATED with: ambulation, deep breathing exercises, incentive spirometry (IS), coughing, adequate pain control so client CAN breathe deeply; WATER (POD 3-5): URINARY TRACT INFECTION — from Foley catheter; increased urinary frequency, urgency, dysuria, cloudy urine; prevent with early Foley removal; WOUND (POD 5-7): WOUND INFECTION — surgical site infection; redness, warmth, swelling, purulent drainage; WALKING (POD 5+): DEEP VEIN THROMBOSIS (DVT)/PULMONARY EMBOLISM — calf pain, swelling, tachycardia, dyspnea; prevent with early ambulation, SCDs, anticoagulants; WONDER DRUGS (any time): DRUG FEVER — particularly from medications like antibiotics, phenytoin, allopurinol; diagnosis of exclusion. INCENTIVE SPIROMETRY (IS): device that provides visual feedback on inspiratory effort; goal is maximum sustained inspiration; hold breath at peak for 2-3 seconds; typical post-op goal: 10-15 breaths per hour while awake; must be used CORRECTLY — slow, deep breath; many clients use it incorrectly as a blowing (expiratory) device when it requires INHALATION. PN ROLE: teach and reinforce IS use pre-operatively (before surgery — better compliance than teaching when in pain); encourage early ambulation (most powerful preventive measure for multiple complications); assess lung sounds for crackles; administer antipyretics and analgesics as ordered; document and report persistent or high fever.