**Core Concept**
Rifampicin resistance in Mycobacterium tuberculosis is primarily due to mutations in the rpoB gene, which encodes the beta-subunit of the RNA polymerase enzyme. This enzyme is essential for DNA-dependent RNA synthesis, and rifampicin exerts its bactericidal effect by inhibiting the RNA polymerase, thereby blocking RNA synthesis.

**Why the Correct Answer is Right**
The rpoB gene is a critical target for rifampicin, as mutations in this gene can lead to changes in the amino acid sequence of the beta-subunit of RNA polymerase. Specifically, mutations at codons 516, 518, and 522 in the rpoB gene are associated with rifampicin resistance. These mutations alter the binding site of rifampicin on the RNA polymerase, preventing the drug from exerting its inhibitory effect.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as the rpsL gene is associated with resistance to streptomycin, not rifampicin.
**Option B:** Incorrect, as the gyrA gene is involved in resistance to fluoroquinolones, not rifampicin.
**Option C:** Incorrect, as the katG gene is responsible for resistance to isoniazid, not rifampicin.

**Clinical Pearl / High-Yield Fact**
Mycobacterium tuberculosis strains resistant to rifampicin are often multidrug-resistant (MDR-TB) and pose a significant challenge in tuberculosis treatment. Early detection of rifampicin resistance is crucial to prevent the development of MDR-TB.

**Correct Answer:** rpoB. The rpoB gene is responsible for resistance to rifampicin.