Using a basic selection model: if 1 in 10^6 bacteria spontaneously carries a resistance mutation and a patient has 10^9 bacterial cells in an infection, estimate how many resistant mutants are expected to be present before treatment begins. Based on that, analyze the implication for single-drug therapy.

• A. Approximately 0.001 resistant mutants expected, meaning resistance will not be an issue and single-drug therapy is safe in most large infections
• B. Approximately 1,000 resistant mutants expected; this suggests single-drug therapy risks rapid selection of resistance because resistant individuals pre-exist in large infections
• C. No resistant mutants expected unless antibiotics are present; spontaneous mutations never occur in the absence of selective pressure
• D. Approximately 10^15 resistant mutants expected because mutation rates multiply with bacterial load, indicating every therapy is futile

Answer: (B)