The "gray matter of the brain is largely made up of which of the following?

Urea is a nitrogenous waste product that is produced when the liver breaks down proteins and amino acids. It is then excreted from the body via sweat.

Psoriasis is a chronic skin condition that causes red, scaly patches on the skin. It is not contagious, and it is not caused by any microorganism. It is believed to be an autoimmune disorder where the immune system attacks healthy skin cells.

Both prokaryotic and eukaryotic cells have a cell membrane. The cell membrane is a barrier that surrounds the cell and separates its internal environment from the external environment.

Browning of the apple surface is due to the action of the enzyme polyphenol oxidase. Lemon juice is acidic and lowers the pH of the apple's surface. This inactivates the enzyme polyphenol oxidase, which works best at a neutral pH. This in turn reduces the browning action on the apple.

Biological macromolecules are large organic molecules such as carbohydrates, lipids, nucleic acids, and proteins. They are built from smaller molecules, which are linked commonly by covalent bonds, and these can be broken down.

The digestive system breaks down complex food substances into small and less complex absorbable compounds. Proteins, for instance, are broken down to constituent amino acids through a digestion process that combines mastication and enzymatic breakdown.

A chlorine molecule contains the fewest covalent bonds among the options provided. It consists of two chlorine atoms bonded together by a single covalent bond. Compared with the other options, ammonia has three covalent bonds, methane has 4 while water has 2.

Most of the carbon dioxide from the blood moves into the alveoli through the process of diffusion down a concentration gradient. In the lungs, there is a higher concentration of carbon dioxide in the blood compared to the alveoli. Its molecules, therefore, move from an area of higher concentration (blood) to an area of lower concentration (alveoli) until equilibrium is reached. This is a passive process and does not require energy or carrier proteins.

Uracil is the nucleotide that is only found in RNA. The other nucleotides are found in both DNA and RNA

The endocardium lines the interior structures of the heart. It is a thin, innermost layer of the heart wall that lines the heart chambers and valves. It is composed of subendocardial connective tissue and endothelial cells that are continuous with blood vessel endothelium.