**Core Concept**
Western Blot is a laboratory technique used for detecting and analyzing proteins in a sample. It involves separating proteins by size using gel electrophoresis, transferring them to a membrane, and then detecting the presence of specific proteins using antibodies.

**Why the Correct Answer is Right**
The correct answer is that Western Blot detects specific proteins in a sample. This is achieved through the use of antibodies that are designed to bind specifically to a particular protein. The antibodies are typically labeled with a marker, such as an enzyme, that produces a detectable signal when the antibody binds to its target protein. This signal can then be visualized using a substrate that reacts with the enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Western Blot is not primarily used for detecting nucleic acids, such as DNA or RNA. While Western Blot can be used to detect proteins that are involved in the regulation of gene expression, it is not a direct method for detecting nucleic acids.
**Option B:** This option is incorrect because PCR (Polymerase Chain Reaction) is a laboratory technique used for amplifying specific DNA sequences, not detecting proteins.
**Option C:** This option is incorrect because ELISA (Enzyme-Linked Immunosorbent Assay) is a laboratory technique used for detecting and quantifying the presence of specific antibodies or antigens in a sample, but it is not the same as Western Blot.

**Clinical Pearl / High-Yield Fact**
Western Blot is a highly specific and sensitive technique for detecting proteins, but it requires careful optimization of the protocol and the use of high-quality reagents to ensure accurate results.

**Correct Answer: D. Specific proteins in a sample.**