B. Systemic response to surgical trauma: This is the most common cause of postoperative pyrexia, especially within the first 48 hours after surgery. The body's response to surgical trauma can lead to a systemic inflammatory response, which may manifest as fever.

C. Basal atelectasis: Atelectasis, or the collapse of lung tissue, is a common postoperative complication, especially after abdominal surgery. It can lead to fever and is consistent with the patient's quiet lung bases on examination.
E. Urinary sepsis: Although the patient has a urinary catheter, which increases the risk of urinary tract infection, the fact that the urine is concentrated might suggest some degree of dehydration but does not directly indicate sepsis. However, urinary sepsis remains a possible cause of postoperative pyrexia.

A. Epidural abscess: Although less likely, an epidural abscess is a serious and potentially life-threatening condition that can occur postoperatively, especially if epidural anesthesia was used. It requires prompt diagnosis and treatment.
D. Infective exacerbation of chronic airways disease: Given the patient's history of chronic airways disease, an infective exacerbation could lead to pyrexia. Although the patient is not in respiratory distress, this condition could rapidly deteriorate and is important not to miss.

Other rare causes of postoperative pyrexia might include deep vein thrombosis, pulmonary embolism, or reactions to medications or transfusions. However, these are less directly related to the information provided in the scenario and would be considered based on additional clinical findings or laboratory results.