Read the following symptoms and suggest the investigation to be done:
1. Salty sweat; many parents notice a salty taste when kissing their child.
2. Poor growth and weight gain (failure to thrive).
3. Constant coughing and wheezing.
4. Thick mucus or phlegm.
5. Greasy, smelly stools that are bulky and pale colored.
Cystic fibrosis is an inherited, chronic, multisystem, life-shortening disorder characterized primarily by poor digestion and obstruction and infection of the airways. 
It is autosomal-recessive and is caused by mutations in both copies of the cystic fibrosis transmembrane regulator gene (CFTR).
The defective exocrine gland function leads to micro-obstruction of the pancreas, which results in cystic degeneration of the pancreas and, ultimately, a digestive enzyme deficiency producing malabsorption of nutrients.
The defective gene products cause abnormal water and electrolyte transport across epithelial cells, which results in a chronic disease of the respiratory and gastrointestinal system, elevated levels of electrolytes in sweat, and impaired reproductive function.

Ref: Mc Donalds, 10th edition, pg 533
CFTR gene mutation analysis
PSA
Karyotype
Transrectal ultrasound