Antitubercular drug causing ophthalmic toxicity is:
**Core Concept**
Ethambutol, a first-line antitubercular agent, is known for its potential to cause ophthalmic toxicity, specifically optic neuritis. This is attributed to its mechanism as an inhibitor of the enzyme arabinosyl transferase, crucial for mycobacterial cell wall synthesis.
**Why the Correct Answer is Right**
Ethambutol's ophthalmic toxicity is mediated through its effect on the optic nerve, leading to impaired color vision and visual acuity. The drug's ability to cause optic neuritis is dose-dependent, with higher doses increasing the risk of ocular toxicity. The underlying pathophysiology involves the inhibition of arabinosyl transferase, which affects the synthesis of the mycobacterial cell wall.
**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid is not commonly associated with ophthalmic toxicity. While isoniazid can cause peripheral neuropathy due to pyridoxine deficiency, it is not a known cause of optic neuritis.
**Option B:** Rifampicin is not typically associated with ophthalmic toxicity. Its primary side effects include hepatotoxicity and gastrointestinal disturbances.
**Option C:** Pyrazinamide is not commonly linked to ophthalmic toxicity. Its primary side effects include hepatotoxicity and arthralgias.
**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients receiving ethambutol for signs of ophthalmic toxicity, including visual disturbances and color vision impairment. Regular visual acuity testing and color vision assessments can help identify potential issues early on.
**Correct Answer: C. Pyrazinamide is not commonly linked to ophthalmic toxicity. Its primary side effects include hepatotoxicity and arthralgias.