## **Core Concept**
The question targets the treatment of infections caused by anaerobic streptococci, which are Gram-positive bacteria that thrive in low-oxygen environments. Effective management of these infections requires antibiotics that are active against anaerobic bacteria.

## **Why the Correct Answer is Right**
Penicillin (option ) is a commonly used antibiotic for treating infections caused by anaerobic streptococci, as these bacteria are typically susceptible to penicillin. Other antibiotics like ceftriaxone (option ) and metronidazole (option ) also have efficacy against anaerobic bacteria. However, aminoglycosides, such as gentamicin (option ), are generally ineffective against anaerobic bacteria because they require oxygen to exert their bactericidal effect.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because penicillin is effective against anaerobic streptococci.
* **Option B:** Incorrect because ceftriaxone, a third-generation cephalosporin, has broad-spectrum activity including anaerobic bacteria.
* **Option C:** Incorrect because metronidazole is a first-line treatment for infections caused by anaerobic bacteria.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that aminoglycosides are ineffective against anaerobic infections. This is crucial because it guides the choice of antibiotics in clinical practice, particularly in severe infections where anaerobic bacteria might be involved.

## **Correct Answer:** . Gentamicin