**Core Concept**
The patient's symptoms of deafness and tinnitus are likely due to ototoxicity, a well-known side effect of certain medications, including anti-tubercular drugs. Ototoxicity refers to the damage caused to the inner ear structures, particularly the cochlea and vestibular apparatus, leading to hearing impairment and balance disturbances.

**Why the Correct Answer is Right**
Streptomycin, an aminoglycoside antibiotic commonly used in anti-tubercular therapy, is known to cause ototoxicity. This occurs through the inhibition of mitochondrial protein synthesis, leading to damage to the hair cells in the cochlea. The patient's symptoms, deafness, and tinnitus in one ear are consistent with the ototoxic effects of streptomycin. The drug's mechanism of action involves binding to the 30S subunit of the bacterial ribosome, but its ototoxic effects are thought to be mediated through its interaction with the mitochondrial ribosomes in the inner ear.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid is a key anti-tubercular drug, but it is not commonly associated with ototoxicity. Its side effects include peripheral neuropathy and hepatotoxicity.
**Option B:** Rifampicin is another important anti-tubercular drug, but it is not known to cause ototoxicity. Its side effects include hepatotoxicity and thrombocytopenia.
**Option C:** Ethambutol can cause optic neuritis, leading to visual disturbances, but it is not associated with ototoxicity. Its side effects include visual disturbances, gait disturbances, and peripheral neuropathy.

**Clinical Pearl / High-Yield Fact**
Aminoglycoside antibiotics, like streptomycin, are ototoxic and nephrotoxic, making it essential to monitor patients for these side effects, particularly in the elderly and those with pre-existing renal impairment.

**Correct Answer: A. Streptomycin**