Examination of an 11-year-old boy revealed frequent nosebleeds, fatigue when walking, underdevelopment of the lower half of the body, increased blood pressure in the upper extremities and decreased pressure in the lower ones, extension of the left heart border, blowing systolic murmur in the interscapular region. ECG shows the horizontal axis of heart. Radiography reveals left cardiomegaly, costal usuration. What is the most likely diagnosis?