<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 5-day-old boy is brought to see his pediatrician after his newborn blood screening showed elevated levels of immunoreactive trypsinogen, a marker for cystic fibrosis. The boy was born at 39 weeks gestation after regular prenatal care. He has 2 siblings that tested negative on screening. On physical exam, his vitals are normal and he appears healthy. Which of the following tests should be performed next to evaluate the newborn for cystic fibrosis?? A: Intestinal current measurement, B: Sweat test, C: Mutation analysis, D: Measurement of fecal elastase levels, E: Nasal potential difference,  <response>: This is a question asking about the next best step in evaluating a newborn with an elevated immunoreactive trypsinogen level on newborn screening, concerning for cystic fibrosis. The key information in the stem is that the baby has an elevated IRT level but is otherwise asymptomatic. Choice A, intestinal current measurement, is not used to diagnose cystic fibrosis. Choice B, the sweat test, is the gold standard for diagnosing cystic fibrosis. It measures chloride levels in sweat, which are elevated in CF due to dysfunction of the CFTR chloride channel. This would be the best next step.Choice C, mutation analysis, can help confirm a diagnosis of CF but is not the immediate next step when there is only an elevated IRT. It could be done later to identify the causative mutations.Choice D, fecal elastase, is not helpful in diagnosing CF in a newborn. It is used later to monitor pancreatic function. Choice E, nasal potential difference, can help diagnose CF but is more complex and not typically used as the next step after newborn screening.Therefore, the sweat test is the best answer as the standard diagnostic test in an asymptomatic baby with an elevated IRT concerning for cystic fibrosis.B: Sweat test 