A 4-year-old Standardbred stallion is being evaluated because he has been racing poorly for the past two months. On auscultation of the thorax, there is an irregular rate and intensity of heart sounds. An electrocardiogram shows an absence of P waves. Which of the following is the most likely diagnosis?
a. Atrial fibrillation
b. Paroxysmal ventricular tachycardia
c. Premature ventricular contractions
d. Ventricular fibrilation
e. Wandering atrial pacemaker