This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
Neuroendocrine cells in the lungs are: 
Select and explain. 
A. Dendritic cells, B. Type I pneumocytes, C. Type II pneumocytes, D. APUD cells
D: APUD cells
Answer: D: APUD cells 
Explanation: APUD (amine precursor uptake and decarboxylation) cells are neuroendocrine cells found in respiratory system, G.I. tract and other organs; they manufactrure amines (e.g. 5-HT). These cells form carcinoid tumors.