Final answer:
Heart failure is a cardiac abnormality seen in approximately a third of anorexia nervosa patients, resulting from extreme weight loss and associated health issues.
Step-by-step explanation:
The cardiac abnormality that can be seen in approximately a third of patients with anorexia nervosa is heart failure. This severe complication arises due to the negative health outcomes associated with anorexia, such as extreme weight loss, bone mass reduction, muscle weakening, and a significant drop in heart rate and blood pressure. Those affected may experience symptoms like weakness, dizziness, and fainting, which can be precursors to heart failure and potentially lead to death. It is crucial for individuals with anorexia nervosa to receive appropriate medical care to mitigate these risks.