**Core Concept**
Streptomycin is an aminoglycoside antibiotic that works by inhibiting protein synthesis in bacteria. It binds to the 30S subunit of the bacterial ribosome, interfering with the initiation complex and preventing the aminoacyl-tRNA from binding to the A site.

**Why the Correct Answer is Right**
The most serious side effect of streptomycin is ototoxicity, specifically vestibulotoxicity. This is due to the drug's ability to damage the hair cells in the vestibular system of the inner ear, leading to vertigo, ataxia, and tinnitus. The mechanism involves the disruption of the endolymphatic potential and the release of calcium ions, which activates the hair cells and leads to their damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Nephrotoxicity is a common side effect of many aminoglycosides, including gentamicin and tobramycin, but it is not the most serious side effect of streptomycin.
**Option B:** Allergic reactions, such as anaphylaxis, are possible with streptomycin but are not the most serious side effect.
**Option C:** Gastrointestinal side effects, such as nausea and vomiting, are common with streptomycin but are not the most serious side effect.

**Clinical Pearl / High-Yield Fact**
Aminoglycosides like streptomycin should be used with caution in patients with pre-existing hearing loss or tinnitus, as these conditions can worsen with prolonged use.

**Correct Answer:** C. Ototoxicity.