What is the most likely etiology of respiratory distress in an infant with chest x-ray showing a "radio-opaque line of fluid in the horizontal fissure"?

The presence of a "radio-opaque line of fluid in the horizontal fissure" on a chest x-ray is indicative of fluid accumulation, which is commonly associated with transient tachypnea of the newborn (TTN). TTN often presents shortly after birth, particularly in infants delivered by cesarean section without preceding labor, and is characterized by respiratory distress due to retained fetal lung fluid.

In TTN, the fluid in the lungs is typically in the interstitial space and can be visible as a line on imaging, particularly affecting the horizontal fissure. This condition results from delayed clearance of lung fluid and generally resolves within a few days. Therefore, the specific radiological finding aligns well with the etiology of TTN, making it the most likely cause of respiratory distress in this context.

Other conditions mentioned may also lead to respiratory distress in infants, but they do not present with the same characteristic radiographic findings. Respiratory distress syndrome, often seen in preterm infants, typically presents with atelectasis rather than fluid lines. Meconium aspiration syndrome could show patchy consolidation or hyperinflation but does not specifically correlate with fluid lines in the horizontal fissure. Pneumonia would generally exhibit more diffuse opacities throughout the lung fields