A patient of multidrug resistant tuberculosis is on antitubercular drugs. After a few months he develops an inability to distinguish between red and green color. Most likely drug causing these symptoms is :
**Core Concept**
The patient's symptoms are likely caused by an antitubercular drug that interferes with the visual pathway, specifically affecting color vision. This is related to the drug's effect on the retina, where photoreceptors are sensitive to different wavelengths of light.
**Why the Correct Answer is Right**
The correct answer is a drug that is known to cause optic neuritis, which affects the optic nerve and can lead to color vision deficiency. Ethambutol, a first-line antitubercular medication, is well-documented to cause this side effect. Ethambutol works by inhibiting the enzyme arabinosyl transferase, which is involved in the synthesis of mycolic acid in Mycobacterium tuberculosis. The optic neuritis caused by ethambutol is thought to be due to its toxic effects on the retinal ganglion cells. Regular monitoring of visual acuity and color vision is required when patients are on ethambutol.
**Why Each Wrong Option is Incorrect**
* **Option A:** Isoniazid is a first-line antitubercular drug that is associated with peripheral neuropathy, hepatitis, and other side effects, but not specifically with color vision deficiency.
* **Option B:** Pyrazinamide is another first-line antitubercular drug that is associated with liver toxicity, arthralgias, and other side effects, but not specifically with color vision deficiency.
* **Option C:** Rifampicin is a first-line antitubercular drug that is associated with hepatotoxicity, orange discoloration of body fluids, and other side effects, but not specifically with color vision deficiency.
**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients on ethambutol for signs of optic neuritis, including color vision deficiency, and to adjust the dosage or discontinue the drug if visual disturbances occur.
**Correct Answer:** C. Ethambutol.