Dextrose is a simple sugar, also known as glucose, that is a primary source of energy for living organisms. It is a monosaccharide, meaning it consists of a single sugar molecule. Dextrose is commonly used in food products as a sweetener and in medical settings as a source of rapidly available energy, such as in intravenous fluids for patients who are unable to eat.
There is no Glucose in D50 there is 25g of dectrose. otherwise known as 50% dextrose in water
You need a minimum of one yeast organism (which is microscopic). If you start with a very small amount of sugar, water, and yeast, you will wind up with a very small amount of ethanol. It's very logical.