**Core Concept**
Warfarin is an anticoagulant that works by inhibiting the synthesis of vitamin K-dependent clotting factors in the liver. The effect of warfarin can be altered by several drugs, including rifampicin, which induces the cytochrome P450 enzyme system, leading to increased metabolism of warfarin and reduced anticoagulant effect.

**Why the Correct Answer is Right**
The patient is on rifampicin, which can induce the enzymes responsible for warfarin metabolism, leading to reduced warfarin levels and decreased anticoagulant effect. This is a classic example of drug-drug interaction, where the effect of one drug (warfarin) is altered by another drug (rifampicin). The patient's prothrombin time is not raised, indicating that the anticoagulant effect of warfarin is diminished.

**Why Each Wrong Option is Incorrect**
* **Option A:** Monitoring prothrombin time alone may not be sufficient to assess the anticoagulant effect of warfarin in this patient, as the effect of rifampicin may not be immediately apparent.
* **Option B:** Switching to a different anticoagulant may not be necessary, as warfarin can still be effective once the rifampicin effect is taken into account.
* **Option D:** Stopping rifampicin may not be feasible or necessary, as it is important for the treatment of tuberculosis.

**Clinical Pearl / High-Yield Fact**
When managing patients on multiple medications, it is essential to consider potential drug-drug interactions, which can significantly alter the efficacy or safety of anticoagulant therapy.

**Correct Answer:** C.