Okay, let's tackle this question. The user provided a question about an anti-tubercular drug that causes contact lens staining. The correct answer is missing, but I need to figure out which drug that is.

First, I remember that certain medications can cause discoloration of contact lenses. Common culprits include drugs that have high pigment content or are excreted in tears. Anti-tubercular drugs like rifampin come to mind because they can cause orange discoloration of bodily secretions, including tears. This staining is due to the drug's metabolites, which are excreted in tears and can deposit on contact lenses, leading to a brownish or orange stain.

Now, looking at the options, the correct answer should be rifampin. Let me verify. Rifampin is known for causing this effect. Other anti-tubercular drugs like isoniazid, pyrazinamide, and ethambutol don't have this side effect. So the correct answer is likely rifampin.

For the explanation: The core concept is the pharmacological property of rifampin leading to contact lens staining. The correct answer explanation would detail how rifampin's metabolites are excreted in tears and bind to the lenses. The wrong options should be explained as not having this effect. The clinical pearl would be to advise patients using contact lenses to avoid rifampin or use alternative medications to prevent staining.

**Core Concept**
This question tests knowledge of drug-induced contact lens staining, specifically related to anti-tubercular therapy (ATT). Rifampin, a first-line ATT drug, is notorious for causing orange-brown discoloration of contact lenses due to its excretion in tears.

**Why the Correct Answer is Right**
Rifampin is a bactericidal antibiotic that inhibits bacterial RNA polymerase. Its metabolites (e.g., 2-isomer of rifampin) are excreted in tears and bind to proteins in contact lenses, causing irreversible staining. This effect is unique to rifampin among ATT drugs and is unrelated to its pharmacological action against *Mycobacterium tuberculosis*.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid does not cause contact lens staining. It primarily causes hepatotoxicity and peripheral neuropathy.
**Option B:** Pyrazinamide lacks pigmented metabolites and has no reported contact lens interactions.
**Option C:** Ethambutol causes optic neuritis but not lens staining.

**Clinical Pearl / High-Yield Fact**
Always warn patients on rifampin about potential contact lens discoloration. Advise temporary lens discontinuation or switch to spectacles during treatment. This staining is dose-dependent and irreversible, making it a classic exam trap.

**Correct Answer: C. Rifampin**

✓ Correct Answer: B. Rifamipicin