**Core Concept**
First-generation cephalosporins are a class of beta-lactam antibiotics characterized by their effectiveness against Gram-positive bacteria, particularly those that cause skin and soft tissue infections. They generally exhibit poor activity against Gram-negative bacteria and are often not active against Pseudomonas aeruginosa.

**Why the Correct Answer is Right**
Cefaclor is a second-generation cephalosporin, whereas the other options (Cefadroxil, Cefazolin, and Cephalexin) are classified as first-generation cephalosporins. Cefaclor has a broader spectrum of activity, including some Gram-negative bacteria, and is more active against Pseudomonas aeruginosa compared to first-generation cephalosporins. Its improved pharmacokinetics and broader spectrum of activity make it a second-generation cephalosporin.

**Why Each Wrong Option is Incorrect**
**Option A:** Cefadroxil is a first-generation cephalosporin that is effective against Gram-positive bacteria and is often used to treat urinary tract infections and skin infections.
**Option B:** Cefazolin is a first-generation cephalosporin that is primarily used in surgical prophylaxis and to treat skin and soft tissue infections.
**Option C:** Cephalexin is a first-generation cephalosporin that is commonly used to treat skin and soft tissue infections, urinary tract infections, and respiratory tract infections.

**Clinical Pearl / High-Yield Fact**
It is essential to note that while first-generation cephalosporins are generally effective against Gram-positive bacteria, they are often not the preferred choice due to their limited activity against Gram-negative bacteria and the presence of beta-lactamase enzymes in some Gram-negative bacteria.

**✓ Correct Answer: D. Cefaclor**