Which of the following factors could lead to loss of eligibility for a first class medical certificate?

Bipolar disorder is a significant mental health condition that can affect an individual's ability to obtain and maintain a first-class medical certificate. The Federal Aviation Administration (FAA) has specific guidelines regarding psychiatric disorders, and bipolar disorder falls under a category that can lead to loss of eligibility due to its potential impact on cognitive function, mood stability, and overall flight safety. Pilots with a history of this condition may be required to demonstrate stability and effective management over a period of time before being considered for certification.

In contrast, the other factors listed are generally viewed more favorably in relation to medical certification. Successful management of stress and excellent physical fitness typically contribute positively to a pilot's overall health and ability to operate an aircraft safely. Controlled diabetes can be managed to a level that does not impair flying ability, allowing individuals with this condition to still be eligible for a medical certificate, depending on how well it is regulated and monitored.