**Core Concept**
Pyridoxine (Vitamin B6) plays a crucial role in various bodily functions, including neurotransmitter synthesis and red blood cell production. Its deficiency can lead to neurological symptoms and hematological abnormalities.

**Why the Correct Answer is Right**
Isoniazid (INH) therapy is known to cause pyridoxine deficiency due to its interference with the metabolism of pyridoxine. INH induces the enzyme inactivating pyridoxine, leading to decreased levels of this vitamin in the body. This deficiency can manifest as peripheral neuropathy, ataxia, or other neurological symptoms. To prevent this condition, pyridoxine supplementation is often recommended for patients on INH therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) deficiency is associated with beriberi and Wernicke-Korsakoff syndrome, which are not directly related to INH therapy.

**Option C:** Folic acid deficiency can occur in patients with malabsorption syndromes, pregnancy, or taking certain medications like methotrexate, but it is not specifically linked to INH therapy.

**Option D:** Cyanocobalamin (Vitamin B12) deficiency is associated with megaloblastic anemia and neurological symptoms, but it is not directly related to INH therapy.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients on INH therapy for signs of pyridoxine deficiency and consider supplementation to prevent peripheral neuropathy and other neurological complications.

✓ Correct Answer: B. Pyridoxine