The diagnosis of rheumatological diseases is challenging among older patients with multimorbidity. Rheumatological diseases in older patients show varied symptoms, such as fatigue, fever, and appetite loss. We encountered an older woman with anti-neutrophil cytoplasmic antibody (ANCA)-related vasculitis complicated by cytomegalovirus (CMV) infection. The case was further complicated by hematochezia and was eventually diagnosed as CMV infection with adverse reactions to medications. This case highlights the difficulty of diagnosing ANCA-related vasculitis and dealing with the complications arising due to the side effects of therapy.