This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Organism causing biliary tract obstruction: 
Select and explain. 
A. Clonorchis sinensis, B. Ankylostoma duodenale, C. Strongyloides stericoralis, D. Enterobius vermicularis
A: Clonorchis sinensis
Answer: A: Clonorchis sinensis 
Explanation: Ans. (a) Clonorchis sinensis