Final answer:
Atrial fibrillation leads to impaired diastolic filling due to loss of the atrial kick, critically reducing ventricular filling and stroke volume, especially in the presence of aortic stenosis, further diminishing cardiac output.
Step-by-step explanation:
Atrial fibrillation is detrimental to cardiac output in patients with aortic stenosis because it leads to impaired diastolic filling. During atrial fibrillation, the atria do not contract effectively, removing the atrial kick, which normally contributes to the last portion of ventricular filling. This is particularly problematic in aortic stenosis, where increased afterload from the stenotic aortic valve already challenges the heart. Without the additional blood volume provided by the atrial kick, the ventricles have less blood to pump out, leading to a decreased stroke volume and subsequently reduced cardiac output.
The correct answer to the student's question is a) Impaired diastolic filling.