In the context of susceptibility interpretation, Narrow corresponds to which set of patterns?

• A. S/R, I/R, R/R
• B. S/S, S/I, I,I
• C. S/I, I/R, R/S
• D. S/R, S/S, I/I

Answer: (A)

Understanding susceptibility patterns helps you see whether an antibiotic is narrow-spectrum, meaning it only works on a limited range of bacteria. A narrow-spectrum agent won’t show broad effectiveness across a diverse panel; you’ll see limited or inconsistent susceptibility and, often, resistance in many tests. The pattern set where every test indicates resistance in the second position (S/R, I/R, R/R) shows that the agent fails against most strains on that panel, reflecting restricted activity characteristic of a narrow-spectrum drug. In contrast, sets with many S or I results and little or no resistance across the panel imply broader activity and are not consistent with a narrow-spectrum interpretation. So this set, with the second-position resistance across all patterns, best fits the idea of a narrow-spectrum interpretation.