A 50 year old male is taking medication for tuberculosis since the last 2 months comes to the OPD with a complaint of hearing problems. The drug with which of the following mechanisms of action is most likely causing these symptoms?
First, I need to recall which anti-TB drugs are known for ototoxicity. The main ones I remember are aminoglycosides like streptomycin and kanamycin. These are used in TB treatment, especially in multidrug-resistant cases. Their mechanism is inhibiting bacterial protein synthesis by binding to the 30S ribosomal subunit. So the correct answer is likely an aminoglycoside.
Now, looking at the options (though they aren't listed here), the correct answer would be the one that mentions inhibiting bacterial protein synthesis via 30S ribosome. The other options might be about other mechanisms. For example, rifampin inhibits RNA polymerase, isoniazid affects mycolic acid synthesis, pyrazinamide's mechanism is less clear but it's thought to act in acidic environments. These don't cause ototoxicity.
Wait, but ethambutol causes optic neuropathy, not hearing issues. So the wrong options might include these other drugs. The key here is the ototoxic side effect pointing to aminoglycosides. So the correct answer's mechanism is binding to 30S ribosome, leading to ototoxicity. The incorrect options would be the other TB drugs with different mechanisms. I should explain each wrong option's mechanism and why they don't cause hearing problems.
**Core Concept** Ototoxicity is a known adverse effect of aminoglycoside antibiotics, which are used in multidrug-resistant tuberculosis treatment. These drugs bind to the 30S bacterial ribosomal subunit, inhibiting protein synthesis and causing cochlear/vestibular damage.
**Why the Correct Answer is Right** Aminoglycosides like streptomycin and kanamycin irreversibly bind to the 30S ribosomal subunit, preventing bacterial protein synthesis. They also cross the blood-labyrinth barrier, leading to oxidative stress and damage to hair cells in the cochlea and vestibular system. This results in sensorineural hearing loss and vertigo, typically dose-dependent and irreversible.
**Why Each Wrong Option is Incorrect**
**Option A:** Rifampin inhibits bacterial RNA polymerase, causing flu-like symptoms and hepatotoxicity, not ototoxicity.
**Option B:** Isoniazid inhibits mycolic acid synthesis in mycobacteria and causes peripheral neuropathy, not hearing loss.
**Option D:** Pyrazinamide disrupts bacterial metabolism in acidic environments and is linked to hyperuricemia, not ototoxicity.
**Clinical Pearl / High-Yield Fact** Aminoglycosides (e.g., streptomycin) are the only first-line TB drugs with ototoxic potential. Always monitor auditory function in patients on these agents. Remember the mnemonic: **"Streptomycin Sings, Kanamycin Kicks, and Both Cause Cochlear Damage."**
**Correct Answer: C. Inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit**