**Core Concept**
The synergistic combination of penicillin and an aminoglycoside is required for effective therapy against certain bacteria, particularly those with high-level resistance to one of the antibiotics. This principle is based on the concept of bactericidal synergy, where the combination of two antibiotics with different mechanisms of action leads to a more effective killing of the bacteria.

**Why the Correct Answer is Right**
The correct answer is **Streptococcus pneumoniae**. This bacterium is a common cause of bacterial meningitis and sepsis, particularly in immunocompromised individuals. The synergistic combination of penicillin (a beta-lactam antibiotic that inhibits cell wall synthesis) and an aminoglycoside (such as gentamicin, which inhibits protein synthesis) is required to achieve bactericidal activity against S. pneumoniae, especially in cases of penicillin-resistant strains. The aminoglycoside enhances the bactericidal activity of penicillin by inhibiting the bacterial protein synthesis, thereby making it more difficult for the bacteria to repair the damage caused by penicillin.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus - While S. aureus can be resistant to penicillin, the synergistic combination of penicillin and an aminoglycoside is not typically required for treatment.
* **Option B:** Escherichia coli - E. coli is often susceptible to aminoglycosides, and penicillin is not typically used as a first-line treatment.
* **Option C:** Pseudomonas aeruginosa - P. aeruginosa is often resistant to aminoglycosides and requires other antibiotics, such as fluoroquinolones or carbapenems, for effective treatment.

**Clinical Pearl / High-Yield Fact**
Remember that the synergistic combination of penicillin and an aminoglycoside is particularly useful in the treatment of bacterial meningitis and sepsis caused by S. pneumoniae, especially in cases of penicillin-resistant strains. This combination can be life-saving in these critically ill patients.

**Correct Answer:** C. Pseudomonas aeruginosa