**Core Concept**
The development of multidrug-resistant (MDR) Mycobacterium tuberculosis strains poses a significant challenge in tuberculosis treatment. These strains exhibit resistance to first-line antitubercular drugs, including streptomycin, isoniazid, and rifampicin. Effective management of MDR-TB requires the use of second-line antitubercular agents.

**Why the Correct Answer is Right**
Linezolid is a second-line antitubercular agent that has shown efficacy against MDR-TB strains, including those resistant to streptomycin. It acts by inhibiting protein synthesis through its binding to the 50S subunit of the bacterial ribosome. This mechanism is distinct from that of first-line antitubercular drugs, making it a valuable option for treating MDR-TB.

**Why Each Wrong Option is Incorrect**
* **Option A:** Isoniazid is a first-line antitubercular agent that is often ineffective against MDR-TB strains resistant to streptomycin. Isoniazid works by inhibiting the synthesis of mycolic acid, a key component of the M. tuberculosis cell wall.
* **Option B:** Streptomycin is an aminoglycoside antibiotic that is ineffective against MDR-TB strains resistant to itself. Its mechanism of action involves binding to the 30S subunit of the bacterial ribosome, inhibiting protein synthesis.
* **Option D:** Ethambutol is a first-line antitubercular agent that is often used in combination with other drugs to treat TB. However, it is not effective against MDR-TB strains resistant to streptomycin. Ethambutol works by inhibiting the synthesis of arabinogalactan, a component of the M. tuberculosis cell wall.

**Clinical Pearl / High-Yield Fact**
Linezolid is a second-line antitubercular agent that requires careful monitoring of blood counts and liver function due to its potential to cause hematologic and hepatotoxic effects.

**Correct Answer:** C. Linezolid