A 25-year-old woman rushes to an emergency depament because she is afraid she is dying. She is experiencing chest pain, a sensation of choking, nausea, and tingling sensations up and down her arms. When the screening nurse examines the patient, her face is flushed and sweating. Her pulse is 140/min and respirations are 25/min. When the emergency depament physician examines her 15 minutes later, her symptoms are dissipating; her pulse is 100/min, and respirations are 20/min. The ECG is normal, except for some residual tachycardia. Which of the following is the most likely diagnosis?