**Core Concept**
Levodopa is a central nervous system stimulant used in the treatment of Parkinson's disease. Its effectiveness can be influenced by various factors, including drug interactions and nutritional supplements.

**Why the Correct Answer is Right**
Vitamin B6 (part of Vitamin B complex) has been shown to decrease the effectiveness of levodopa by increasing the decarboxylation of levodopa to dopamine outside the brain. This peripheral decarboxylation reduces the amount of levodopa that crosses the blood-brain barrier, thereby decreasing its therapeutic effect. This interaction highlights the importance of considering nutritional supplements when prescribing levodopa.

**Why Each Wrong Option is Incorrect**
**Option A:** Carbidopa is actually a drug that inhibits the peripheral decarboxylation of levodopa, thereby increasing its availability to the brain and enhancing its therapeutic effect.
**Option B:** MAO (monoamine oxidase) inhibitors can increase the levels of dopamine in the brain by preventing its breakdown, thereby potentially enhancing the effect of levodopa rather than decreasing it.
**Option D:** COMT (catechol-O-methyltransferase) inhibitors, such as entacapone, are used in combination with levodopa to decrease its breakdown and increase its availability to the brain, thereby enhancing its therapeutic effect.

**Clinical Pearl / High-Yield Fact**
When prescribing levodopa, it's essential to advise patients to avoid taking vitamin B6 supplements, as this can decrease the effectiveness of the medication.

**✓ Correct Answer: C. Vit. B complex**