A 10-year-old, male-castrated golden retriever has a 1-month history of mild lethargy and decreased appetite. On physical examination, he has pale mucous membranes and weak femoral pulses. His complete blood count (CBC) shows a decreased packed cell volume (PCV), decreased mean corpuscular volume (MCV), and decreased mean corpuscular hemoglobin concentration (MCHC). His biochemistry panel shows a mildly increased blood urea nitrogen (BUN) level and mildly increased serum alkaline phosphatase (ALP) level. Which of the following is the most likely cause of this dog’s anemia?