When is an anti-TB drug considered likely effective?

An anti-TB drug is considered likely effective when it has not been used in a regimen that failed to cure the individual patient. This is crucial because if a drug has already been part of a treatment plan that did not lead to a successful outcome, it may indicate resistance or ineffectiveness specific to that patient. Effectiveness is assessed not just on the drug's general efficacy, but also on its previous role in treatment for the particular patient.

The other options deal with aspects that either present concerns regarding the efficacy of the drug or do not directly indicate effectiveness. For instance, if a close contact is resistant to the drug, this raises concerns about the potential for resistance in others, suggesting that the drug might not be effective. Similarly, if drug susceptibility testing (DST) indicates that the drug is resistant, it eliminates the likelihood of effectiveness altogether. Lastly, while first-line medications are generally more effective against TB, their classification does not guarantee effectiveness in every case, particularly if prior treatment attempts with the drug have failed. Thus, the context of previous use and patient-specific factors is paramount in determining the likelihood of effectiveness.