**Core Concept**
Vitamin B6, also known as pyridoxine, plays a crucial role in various bodily functions, including neurotransmitter synthesis and amino acid metabolism. A deficiency in Vitamin B6 can lead to impaired function of enzymes involved in these processes, resulting in increased excretion of certain substances.

**Why the Correct Answer is Right**
Vitamin B6 is a co-factor for the enzyme cystathionine beta-synthase, which is involved in the conversion of homocysteine to cystathionine. Without sufficient Vitamin B6, this enzyme is impaired, leading to an accumulation of homocysteine in the blood. Homocysteine is then excreted in the urine, making its measurement a useful indicator of Vitamin B6 status.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Vitamin B6 deficiency does not directly affect the excretion of glucose. Glucose metabolism is primarily regulated by insulin and glucagon.
**Option B:** This option is incorrect because Vitamin B6 deficiency does not directly affect the excretion of uric acid. Uric acid metabolism is primarily regulated by the enzyme xanthine oxidase.
**Option C:** This option is incorrect because Vitamin B6 deficiency does not directly affect the excretion of creatinine. Creatinine is a byproduct of muscle metabolism and is primarily excreted by the kidneys regardless of Vitamin B6 status.

**Clinical Pearl / High-Yield Fact**
In patients with Vitamin B6 deficiency, increased excretion of homocysteine can be an early indicator of the deficiency, even before symptoms appear. This is particularly relevant in patients with gastrointestinal disorders that may impair Vitamin B6 absorption.

**Correct Answer:** Homocysteine.