Proteins consist of the elements nitrogen, carbon, hydrogen, and oxygen.  They are polymers of molecules called amino acids.  Lipids are made up of glycerol and fatty acid chains.

All amino acids have an amino group and a carboxyl group.  Amino acids do not have glycerol.  The side chain called an R-group is what differentiates amino acids from each other in their chemical properties and functions.

Primary structure takes advantage of covalent bonding to form a peptide bond between amino acids. Secondary structure takes advantage of hydrogen bonding to form alpha helices and beta-pleated sheets. The tertiary structure takes advantage of all these forms of bonding and interactions to fold into the overall shape of the protein and the quaternary structure is formed by using these interactions to connect multiple polypeptides.

Proteins preform multiple functions in the body. Carrier proteins move molecules from place to place. Sometimes the molecules they carry relay signals between cells. Proteins can bind to substrates to bring the ingredients of key reactions together, catalyzing those reactions. Certain proteins integrate into cell membranes providing structural elements. Fatty acids store energy, while proteins do not. They lack the highly reduced carbon-hydrogen bonds of fatty acids. 

When two substrates compete for the same active site on an enzyme, it is competitive inhibition. Allosteric inhibition occurs when a substrate binds to another part of the enzyme away from the active site. Noncompetitive inhibition is another name for this. Feedback inhibition is a specific situation when the product of a reaction catalyzed by the enzyme comes back to change its function.

Enzymatic proteins are essential to metabolic processes within the cell in that they speed up cellular reactions that would otherwise take too long. Defensive proteins help fight disease, receptor proteins respond to stimuli, motor proteins help with cell movement, and transport proteins move substances across the cell.  

In a fasting state, the body will attempt to break down macromolecules into smaller molecules to help generate ATP. Nitrogen is abundant in proteins, and thus excessive amounts of nitrogen in the urine likely came from the breakdown of proteins secondary to starvation. The other answer choices would not explain the presence of nitrogen in the urine.

Essential amino acids are termed "essential" because humans cannot biologically manufacture them. Therefore, they must be ingested exogenously (from sources outside of the human body). There are ten essential amino acids, and these ten are obtained from the human diet (e.g., foods we ingest)

The ringed structure indicates that the molecule in question is in fact a steroid molecule. For example, cholesterol has is an important steroid molecule in the human body. It contains four rings in its structure. Steroids are easy to identify by their ringed structure.

Chargaff found that in double-stranded DNA, the number of guanine bases should be equal to the number of cytosine bases, and the number of adenine bases should equal the number of thymine bases. These rules proved to be important pieces of evidence for the idea of complementarity, the theory that each DNA base pairs only with a specific other base on its opposite strand.

According to Chargaff's rules, the statement regarding guanine and cytosine bases is correct. The two other statements that are similarly worded are not correct because they do not compare the frequencies of two bases that are complementary to each other (adenine will not bind cytosine and guanine will not bind thymine). Finally, guanine-cytosine bonds are more stable than adenine-thymine bonds.