**Core Concept**
Southern blot is a laboratory technique used to detect specific DNA sequences in a sample. It involves the separation of DNA fragments by size using electrophoresis, transfer of the fragments to a membrane, and hybridization with a labeled probe to visualize the target sequence.

**Why the Correct Answer is Right**
The Southern blot technique is particularly useful for detecting specific genes, mutations, or chromosomal rearrangements in DNA samples. The process involves denaturing the DNA, separating it by size using gel electrophoresis, and then transferring the fragments to a membrane. A labeled probe is then hybridized to the membrane to detect the target sequence. The probe is a short, single-stranded DNA or RNA molecule that is complementary to the target sequence.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Western blot is a technique used to detect specific proteins, not DNA sequences.
**Option B:** This is incorrect because PCR (Polymerase Chain Reaction) is a technique used to amplify specific DNA sequences, but it is not used to detect DNA sequences.
**Option C:** This is incorrect because Northern blot is a technique used to detect specific RNA sequences, not DNA sequences.

**Clinical Pearl / High-Yield Fact**
The Southern blot technique is often used in molecular diagnostics to detect genetic mutations or chromosomal rearrangements associated with genetic disorders.

**Correct Answer: D. Southern blot.**