This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
[{'content': 'True statement regarding diphtheria is \nSelect and explain. \nA. It can be diagnosed by demonstration of antibodies by ELISA, B. Immunisation prevents carrier state, C. Treatment of contacts is not indicated, D. Iron has critical value in the production of toxin', 'role': 'user'}, {'content': "Answer: D \nExplanation: Toxin production is influenced by the concentration of iron in the medium. The optimum level of iron for toxin production is 0.1 mg/l, while a concentration of 0.5mg/l inhibits the formation of the toxin. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 240", 'role': 'assistant'}]