This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nose and PNS, Subject name: ENT
A 10-year-old child presents with history of chronic sinusitis and multiple nasal polypi. He also suffers from recurrent chest infections and malabsorption syndrome. Which one of the following tests would be useful for diagnosis? 
Select and explain. 
A. Total IgE estimation, B. ESR, C. Level of angiotensin converting enzyme, D. Sweat chloride
D: Sweat chloride
Answer: D: Sweat chloride 
Explanation: Presence of chronic sinusitis, multiple nasal polypi and malabsorption point towards cystic fibrosis and therefore sweat chloride test would be useful.