A patient in their early 70s with a history of lung cancer was admitted to the oncology department for routine assessment and next cycle of anticancer therapy. Previously, the patient had undergone a left upper lobectomy followed by 19 doses of pembrolizumab for lung cancer; the 20th dose was to be administered during this visit. The patient had a medical history of well-controlled diabetes and immune-related hypothyroidism associated with pembrolizumab use, and no other comorbidities or noteworthy family medical history. On admission, the patient denied any symptoms and had normal vital signs. However, the initial electrocardiogram (ECG) findings showed alarming features that alerted the physicians who ordered additional investigations (Figure, A). The patient’s troponin I level was 0.063 ng/mL (reference, <0.034 ng/mL; for µg/L, multiply by 1); brain-type natriuretic peptide level was 197 pg/mL (reference, <125 pg/mL; for ng/L, multiply by 1); and serum calcium level was 8.72 mg/dL (reference range, 8.5-10.5 mg/dL; for mmol/L, multiply by 0.25). Ten minutes after the initial ECG was performed, the patient complained of sudden-onset squeezing precordial chest pain with back pain and nausea.