An 8-month-old girl with Turner syndrome is brought to the emergency room by her parents, who complain that their daughter is breathing rapidly and not eating. Physical examination reveals tachypnea, pallor, absent femoral pulses, and a murmur heard at the left axilla. There is hypertension in the upper extremities and low blood pressure in both legs. A chest X-ray shows notching or scalloping of the ribs. What is the appropriate diagnosis?