**Core Concept**
Streptococcus pneumoniae is a Gram-positive bacterium that causes a range of infections, primarily affecting the respiratory system. The bacterium invades the host by adhering to the epithelial lining and producing capsular polysaccharides, which evade the host's immune response.

**Why the Correct Answer is Right**
The most common infection caused by Streptococcus pneumoniae is pneumonia, particularly community-acquired pneumonia (CAP). This bacterium is a leading cause of CAP, particularly in individuals with underlying health conditions or weakened immune systems. The bacterium's capsular polysaccharides, such as pneumococcal polysaccharide 19F, play a crucial role in evading the host's immune response and causing disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** While Streptococcus pneumoniae can cause otitis media, it is not the most common infection caused by this bacterium.
* **Option B:** Streptococcus pneumoniae can cause bacteremia, but this is a less common presentation compared to pneumonia.
* **Option C:** While Streptococcus pneumoniae can cause meningitis, particularly in children and older adults, this is a less common infection compared to pneumonia.

**Clinical Pearl / High-Yield Fact**
The pneumococcal conjugate vaccine (PCV) has been shown to significantly reduce the incidence of invasive pneumococcal disease, including pneumonia, in children and adults.

**Correct Answer:** C. Pneumonia.