Final answer:
The principal diagnosis for the patient admitted with a hip fracture but unable to undergo surgery due to cardiac arrhythmia is the cardiac arrhythmia, as it is the condition that required the most urgent and extensive medical attention during the hospital stay.
Step-by-step explanation:
The principal diagnosis for a patient who was admitted for a fracture but could not undergo surgery due to the development of a cardiac arrhythmia would be the cardiac arrhythmia because it is the condition that after study occasioned the admission of the patient to the hospital and resulted in the inability to perform the planned surgery. The hip fracture is a significant medical condition, but in this scenario, it is not the principal diagnosis because it is not what led to the most resource-intensive treatment or greatest morbidity management during the hospital stay.
It is important to note that while hip fractures, especially in elderly patients, can lead to severe complications such as blood clots and pneumonia, the immediate concern preventing surgery in this case is the cardiac arrhythmia, which takes precedence as the condition requiring the most immediate and intensive management.
Should the cardiac condition stabilize, attention can then be redirected to address the hip fracture, likely through open reduction and internal fixation to promote healing and mobility.