**Core Concept**
Streptomycin is an aminoglycoside antibiotic that works by inhibiting protein synthesis in bacteria. Its use is associated with several adverse effects, particularly ototoxicity and nephrotoxicity. Ototoxicity is a critical concern as it can lead to permanent hearing loss and balance disorders.

**Why the Correct Answer is Right**
The most serious adverse effect of streptomycin is ototoxicity, primarily affecting the vestibulocochlear nerve. This is due to the drug's ability to accumulate in the inner ear, leading to damage to the hair cells and subsequent hearing loss. The exact mechanism involves the inhibition of mitochondrial protein synthesis, which disrupts the normal functioning of the hair cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Nephrotoxicity is a significant adverse effect of streptomycin, but it is not the most serious. While renal impairment can occur, it is generally reversible with discontinuation of the drug.
**Option B:** Allergic reactions can occur with streptomycin, but they are rare and not as serious as ototoxicity. Anaphylaxis is a potential risk, but it is not the most common adverse effect.
**Option C:** Gastrointestinal disturbances, such as nausea and vomiting, are common side effects of streptomycin. However, they are generally mild and not as severe as ototoxicity.

**Clinical Pearl / High-Yield Fact**
Aminoglycoside antibiotics like streptomycin are often associated with ototoxicity, and the risk is increased in patients with pre-existing hearing loss or kidney disease. It is crucial to monitor patients receiving streptomycin for signs of ototoxicity, such as tinnitus or vertigo.

**Correct Answer:** C. Ototoxicity is the most serious adverse effect of streptomycin.