Ethylhydrocupreine HC1 susceptibility is a presumptive test for the identification of which species?

• A. Viridans streptococci
• B. Streptococcus pneumoniae
• C. Streptococcus pyogenes
• D. Streptococcus agalactiae

Answer: (B)

Ethylhydrocupreine HC1 (optichin) susceptibility is used to separate Streptococcus pneumoniae from other alpha-hemolytic streptococci. Pneumococcus is sensitive to optochin and will show a clear zone of inhibition around the optichin disc on a blood agar plate, indicating its presence. In contrast, viridans streptococci are resistant and do not produce a zone, so they do not look like pneumococcus on this test. This makes the test a quick presumptive tool for identifying Streptococcus pneumoniae among alpha-hemolytic streptococci. The other organisms listed are not identified by optochin susceptibility, as they do not reliably show the optochin-sensitive pattern used to differentiate pneumococcus.