**Core Concept**
Tuberculosis (TB) caused by Mycobacterium tuberculosis is a major public health concern worldwide. Resistance to first-line antitubercular drugs, particularly isoniazid (INH) and rifampicin (RIF), poses a significant challenge in TB management. The emergence of multi-drug resistant TB (MDR-TB) necessitates the use of second-line antitubercular agents.

**Why the Correct Answer is Right**
In cases of MDR-TB, where the causative organism is resistant to both INH and RIF, the most appropriate treatment strategy involves the use of second-line antitubercular drugs. Fluoroquinolones, such as levofloxacin (LFX), have been shown to be effective against M. tuberculosis, including strains resistant to INH and RIF. LFX works by inhibiting bacterial DNA gyrase and topoisomerase IV, essential enzymes for DNA replication and transcription.

**Why Each Wrong Option is Incorrect**
**Option A:** Streptomycin is an aminoglycoside antibiotic that has been used to treat TB, but it is not the most appropriate choice for MDR-TB due to its limited efficacy and potential for resistance development.

**Option B:** Ethambutol is a first-line antitubercular agent, but it is not sufficient as monotherapy for MDR-TB, as the causative organism may still be resistant to it.

**Option C:** Isoniazid and rifampicin are first-line antitubercular agents, but they are ineffective against MDR-TB, making them an inappropriate choice.

**Clinical Pearl / High-Yield Fact**
When treating MDR-TB, it is essential to use a combination of at least four second-line antitubercular agents, including a fluoroquinolone, an injectable agent (such as streptomycin), a second-line injectable agent (such as amikacin), and a single oral agent (such as cycloserine or linezolid).

**Correct Answer:** B.