**Core Concept**
The synergistic effect of combining penicillin and an aminoglycoside is crucial in treating infections caused by certain gram-positive bacteria, particularly those with a high degree of resistance to penicillin. This combination takes advantage of the bactericidal effect of aminoglycosides, which is potentiated by the inhibition of cell wall synthesis caused by penicillin.

**Why the Correct Answer is Right**
The synergistic activity of penicillin and an aminoglycoside is particularly effective against gram-positive coccibacilli, such as Enterococcus faecalis. Penicillin inhibits cell wall synthesis, leading to cell lysis, while aminoglycosides induce protein synthesis inhibition, leading to further cell death. This combination is essential in treating infections where high-level resistance to penicillin is present, such as in endocarditis and urinary tract infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is typically susceptible to penicillin but may develop resistance through the production of beta-lactamases. The synergistic effect of penicillin and an aminoglycoside is not necessary for treating infections caused by this organism.
**Option B:** Streptococcus pneumoniae is generally susceptible to penicillin and does not require the synergistic effect of an aminoglycoside for effective therapy.
**Option C:** Escherichia coli is a gram-negative bacterium that is typically susceptible to aminoglycosides but not penicillin. The synergistic effect of these two antibiotics is not relevant in treating infections caused by this organism.

**Clinical Pearl / High-Yield Fact**
When treating gram-positive infections with high-level resistance to penicillin, such as those caused by Enterococcus faecalis, the combination of penicillin and an aminoglycoside should be used to achieve synergistic bactericidal effects.

**Correct Answer:** C.