**Core Concept**
Northern blotting is a laboratory technique used for the separation and identification of RNA molecules based on their size and sequence. It involves the transfer of RNA from a gel to a membrane, followed by hybridization with a labeled probe to detect specific RNA sequences.

**Why the Correct Answer is Right**
Northern blotting is particularly useful for detecting the expression of specific genes, such as those involved in disease processes. The technique involves the use of a labeled probe that is complementary to the target RNA sequence, allowing for the detection of specific RNA molecules. The hybridization reaction is typically performed under conditions that are specific to the probe and target RNA, ensuring high specificity and sensitivity.

**Why Each Wrong Option is Incorrect**
**Option A:** Southern blotting is a related technique used for DNA, not RNA.
**Option B:** Western blotting is another laboratory technique used for detecting proteins, not RNA.
**Option C:** PCR (Polymerase Chain Reaction) is a technique used for amplifying DNA or RNA sequences, but it is not a separation and diagnosis method like Northern blotting.

**Clinical Pearl / High-Yield Fact**
Northern blotting can be used to detect the expression of genes involved in various diseases, such as cancer or viral infections. It is a valuable tool for understanding gene regulation and expression in different biological contexts.

**Correct Answer: C. RNA**