**Core Concept**
Ceftriaxone is a **third-generation cephalosporin** antibiotic, which works by inhibiting cell wall synthesis in bacteria. It has a broad spectrum of activity against both Gram-positive and Gram-negative bacteria. Ceftriaxone is often used to treat various infections, including those of the skin, respiratory tract, and urinary tract.

**Why the Correct Answer is Right**
Given the broad spectrum of activity and the classification as a **third-generation cephalosporin**, ceftriaxone is effective against a wide range of bacteria, including many strains resistant to earlier generations of cephalosporins. Its mechanism involves binding to **penicillin-binding proteins** and inhibiting the synthesis of the bacterial cell wall, leading to cell lysis and death.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ceftriaxone's spectrum is broader than just Gram-positive bacteria.
**Option B:** Incorrect as it is not limited to just Gram-negative bacteria.
**Option C:** Incorrect because it is not a first-generation cephalosporin, which has a narrower spectrum of activity.

**Clinical Pearl / High-Yield Fact**
Ceftriaxone has a long half-life, allowing for once-daily dosing, and it is often used in outpatient settings for the treatment of community-acquired infections. Remember, **ceftriaxone** is a key option for treating **gonorrhea** due to its efficacy against *Neisseria gonorrhoeae*.

**Correct Answer:** D. A third-generation cephalosporin.