What physiological change is common in a 3-month-old infant with cystic fibrosis?

A common physiological change observed in a 3-month-old infant with cystic fibrosis is the presence of foul-smelling, greasy stools. This symptom occurs due to the impact of non-functioning or insufficient pancreatic enzymes, which are crucial for the digestion and absorption of fats and nutrients. In cystic fibrosis, the thick mucus that clogs the pancreatic ducts hinders these enzymes from reaching the intestines, leading to malabsorption. As a result, the undigested fat is excreted in the stool, making it appear greasy and giving it a foul odor.

This is a significant clinical indicator that healthcare providers look for when diagnosing cystic fibrosis in infants. The other options do not typically align with the expected physiological responses in an infant diagnosed with cystic fibrosis at this age. For example, weight gain may not be observed due to malabsorption issues, while the ability to hold the head upright is a developmental milestone not directly related to cystic fibrosis symptoms. Increased appetite may also not correlate with the clinical picture, as malabsorption often leads to inadequate weight gain rather than an increase in appetite.