A baby boy was born at 2:45 A.M. after a 35-week gestation. He weighed 1170 g. Upon admission to the premature nursery, he had slight respiratory distress, nasal flaring, grunting, intercostal retractions, and slight cyanosis. Apgar score at one minute was 4, and at five minutes, it was 6. Apical pulse is 164, respirations are 44, and axillary temperature is 96°F. What was the most likely cause of the baby's cyanosis?