**Core Concept**
Peripheral neuritis refers to inflammation of the peripheral nerves, leading to damage of the myelin sheath or the axon itself. This can result in a range of symptoms, including pain, numbness, and weakness in the affected limbs.

**Why the Correct Answer is Right**
The correct answer is **C. Isoniazid**. Isoniazid is a first-line antitubercular medication that can cause peripheral neuritis as a side effect. This is due to its interference with vitamin B6 metabolism, leading to a deficiency of pyridoxine. Pyridoxine is essential for the synthesis of neurotransmitters and the maintenance of the myelin sheath. Without sufficient pyridoxine, the peripheral nerves can become damaged, resulting in neuritis.

**Why Each Wrong Option is Incorrect**
**Option A:** **A. Penicillin** - While penicillin can cause allergic reactions, it is not typically associated with peripheral neuritis. **Option B:** **B. Ciprofloxacin** - Although ciprofloxacin can cause neurological side effects, such as seizures and tremors, peripheral neuritis is not a common adverse effect. **Option D:** **D. Ethambutol** - Ethambutol is another antitubercular medication, but it is more commonly associated with optic neuritis, rather than peripheral neuritis.

**Clinical Pearl / High-Yield Fact**
To prevent isoniazid-induced peripheral neuritis, patients should receive pyridoxine supplementation concurrently with isoniazid therapy. This can help maintain adequate pyridoxine levels and reduce the risk of neurological complications.

**Correct Answer:** C. Isoniazid causes peripheral neuritis.