After an infection was treated with a new drug, inflammation decreased - HSC - SSCE Biology - Question 12 - 2008 - Paper 1

The most valid conclusion from the observations is that some pathogens resistant to the drug were already present before the treatment began. Their prevalence may have increased due to the selective pressure exerted by the drug, which killed off the non-resistant strains, allowing the resistant ones to proliferate. This aligns with option (D), which states that a few pathogens resistant to the drugs were present at the start of treatment and natural selection increased their numbers.