**Core Concept**
Streptococcus agalactiae, also known as Group B streptococcus (GBS), is a gram-positive bacterium that is a leading cause of neonatal sepsis and meningitis. **GBS is a commensal organism found in the gastrointestinal and genitourinary tracts of healthy adults and infants.**

**Why the Correct Answer is Right**
GBS can cause severe disease in newborns, particularly those born to mothers who are colonized with the bacteria. **GBS colonizes the maternal genital tract and can be transmitted to the fetus during delivery, leading to invasive disease such as pneumonia, sepsis, and meningitis.** The bacteria can also cause disease in adults, particularly those with underlying medical conditions. **GBS can cause bacteremia, pneumonia, and endocarditis in adults, particularly those with heart conditions.**

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary mode of transmission of GBS.
**Option B:** This option is incorrect because it does not accurately describe the primary disease manifestations of GBS in newborns.
**Option C:** This option is incorrect because it does not accurately describe the primary risk factors for GBS disease in adults.

**Clinical Pearl / High-Yield Fact**
GBS is a leading cause of neonatal sepsis and meningitis, and prevention strategies include screening pregnant women for GBS colonization and administering intrapartum antibiotics to those who are colonized. **GBS disease is a medical emergency that requires prompt recognition and treatment.**

**Correct Answer: D. Streptococcus agalactiae is a leading cause of neonatal sepsis and meningitis.**