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
In a closed system, spores are formed during which of the following phases of bacterial growth? 
Select and explain. 
A. Exponential phase, B. Lag phase, C. Log phase, D. Stationary phase
D: Stationary phase
Answer: D: Stationary phase 
Explanation: Spore formation usually occurs during the stationary phase, when cell growth ceases because of a developing lack of nutrients or accumulation of toxins. The exponential phase and log phase are descriptors for the steady state of active growth occurring after the lag phase and before the stationary phase. Many antibiotics are most effective in this period. The lag phase is the initial period of adaptation, prior to growth, which occurs when organisms are introduced to a new environment. Ref: Ray C.G., Ryan K.J. (2010). Chapter 21. The Nature of Bacteria. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.