For a 3-month-old not gaining weight and with fatty stools, what test should be ordered?

In the case of a 3-month-old experiencing issues with weight gain and exhibiting fatty stools, the most appropriate test to order is fecal fat analysis. This test specifically evaluates the presence of fat in the stool, which can indicate malabsorption issues. Given the symptoms, the presence of fatty stools suggests that the infant is not adequately absorbing nutrients, which could be due to several conditions including cystic fibrosis, pancreatic insufficiency, or other malabsorptive disorders.

Cystic fibrosis is a genetic condition that can cause pancreatic issues leading to malabsorption, and while testing for cystic fibrosis mutations might be relevant in some cases, fecal fat analysis is the more immediate and direct method to assess for malabsorption. It can provide essential information on whether the infant's symptoms are due to dietary absorption issues rather than solely confirming a genetic diagnosis.

Growth hormone levels and serum electrolytes do not directly relate to the symptoms of poor weight gain with fatty stools and would not yield the specific information needed to address potential malabsorption. Thus, ordering fecal fat analysis takes precedence in this scenario to guide appropriate management and further evaluations.