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: Medicine
Which drug is not effective against H. pylori: 
Select and explain. 
A. Colloidal Bismuth, B. Metronidazole, C. Amoxicillin, D. Erythromycin
D: Erythromycin
Answer: D: Erythromycin 
Explanation: Answer is D (Erythromycin): The most successful are the Triple and Quadruple multi-drug regimens: Name Drug 1 Drug 2 Drug 3 Drug 4 OCA Omeprazole (20 mg BD) Clarithromycin (500 mg BD) Amoxicillin (1g BD) OCM Omeprazole (20 mg BD) Clarithromycin (500 mg BD) Metronidazole (500 mg BD) OBTM Omeprazole (20 mg BD) Bismuth Subsalicylate (2 tabs QID) Tetracycline (500 mg Q1D) Metronidazole