What health issue caused Nick's dad to be sent back to the hospital?

Nick's dad was sent back to the hospital due to an untreated infection, which is a serious health concern. Infections can escalate quickly and lead to severe complications if they are not managed appropriately. Common symptoms may include fever, pain, and general malaise, which require medical evaluation and treatment, particularly if the infection could potentially spread or worsen over time.

In the context of this question, untreated infections can cause significant health issues, leading to hospital readmissions as medical professionals are needed to provide intravenous antibiotics, monitor vital signs, and ensure proper recovery.

The other health issues mentioned, while serious in their own regard, do not necessarily imply a return to the hospital in the same urgent manner as an untreated infection can. For instance, a broken leg may not require hospitalization if it can be managed with a cast and outpatient care, dehydration can often be treated on an outpatient basis with fluids, and heart problems could lead to various responses depending on severity and prior care. Thus, the immediacy and potential dangers associated with an untreated infection make it the most compelling reason for hospitalization in this scenario.