Final answer:
The sweat chloride test is the most specific and helpful diagnostic test for cystic fibrosis, as it measures chloride levels in sweat and directly relates to CFTR function.
Step-by-step explanation:
The sweat chloride test is the most helpful at diagnosing a patient with cystic fibrosis. This condition is characterized by a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR, that affects chloride ion transport.
A sweat chloride test measures the amount of chloride in the sweat and is a standard diagnostic tool for cystic fibrosis. In contrast, metabolic studies and calorimetry are less specific to cystic fibrosis, while pulmonary function studies like spirometry can help assess the impact of the disease on lung function but are not diagnostic.