A gene mutation causes the LDL receptor to have a different shape so that it no longer interacts with LDL and receptor-mediated endocytosis is not stimulated to take LDL into cells. LDL then increases in the extracellular fluid, which includes plasma in the bloodstream, causing high blood levels of LDL. LDL is a transport particle for cholesterol, and is known as "bad cholesterol." This is an example of which theme in physiology?

This is an example of "structure determines function".

Explanation:

The structure-function relationship is the most widely occurring theme in physiology. Predominantly, the structure of a particular organ, tissue or cell determines the function it plays in the body.

This is because this structure is solely determined on the basis of the sequence of amino acids in proteins and the nature of the chemical bonds that exist between them.

This is particularly true regarding receptors and cell surface protein. Receptor-ligand interaction work on the principle of the "lock and key model". Both have specific shapes and structures that are compatible with each other. Even the slightest change in structure of the receptor will render it useless in ligand recognition and binding.

Low Density Lipoprotein Receptor (LDLR) is a transmembrane receptor present on the cell surface membrane of hepatocytes and other cells in the body. It is involved in the transport of cholesterol. Like all receptors, the LDLR also possesses a certain structure (dictated by amino acid sequence). If this structure is altered, the receptor will be unable to recognize and bind LDL, causing it to accumulate in the extracellular matrix.