A newborn presents with an early onset of dyspnea with chest retractions, expiratory grunting and cyanosis following an uneventful normal preterm labor. On examination no cardiac murmurs are heard and the lungs appear clear. On a plain X-ray there is evidence of prominent pulmonary vascular markings and fluid lines in the fissures. The cyanosis improves with minimal oxygen. The most probable diagnosis is: