Final answer:
Not passing meconium in a 3-day-old newborn could indicate meconium ileus or Hirschsprung's disease, both of which necessitate immediate medical attention.
Step-by-step explanation:
A 3-day-old newborn that has not passed meconium may be experiencing a condition that is a cause for concern. Two differential diagnoses for this include meconium ileus and Hirschsprung's disease. Meconium ileus is a type of bowel obstruction caused by thick, sticky meconium and is often associated with cystic fibrosis. On the other hand, Hirschsprung's disease is a condition where a portion of the bowel lacks the nerves that stimulate peristalsis, leading to blockage and an inability to pass stool. Both conditions require prompt medical evaluation and treatment.