Final answer:
The inability to pass a small catheter through the nasal cavity in a newborn is most likely indicative of choanal atresia, a congenital blockage of the nasal passage. It differs from nasal polyps and should not be confused with cleft palate, which involves a gap in the hard palate causing communication between the mouth and nose.
Step-by-step explanation:
When evaluating a newborn, the inability to pass a small catheter through the nasal cavity may be most indicative of a condition known as choanal atresia. Choanal atresia is a congenital disorder where the back of the nasal passage is blocked, typically by abnormal bony or soft tissue formation. This condition can be either unilateral or bilateral and is different from nasal polyps, which are noncancerous growths that can also obstruct the nasal passages but are less likely to be present at birth.
Cleft palate is another condition of concern in newborns that affects the structure of the mouth and nasal cavity. However, if a catheter cannot pass through the nasal cavity, choanal atresia is a more probable diagnosis. Cleft palate is characterized by a gap in the hard palate that leads to communication between the mouth and nose and can severely affect an infant's ability to nurse properly.