Choice A reason: Frostbite is caused by exposure to extreme cold, which is a physical factor that can damage the cells. Physical injury can result in cell membrane damage, cytoplasmic swelling, organelle dysfunction, or DNA breakage.

Choice B reason: Chemical injury is caused by exposure to toxic substances, poisons, or pollutants that can damage the cells. Chemical injury can result in cell membrane damage, cytoplasmic swelling, organelle dysfunction, or DNA breakage.

Choice C reason: Biological injury is caused by infection by microorganisms, such as bacteria, viruses, fungi, or parasites that can damage the cells. Biological injury can result in inflammation, immune response, or tissue damage.

Choice D reason: Genetic injury is caused by mutations or alterations in the DNA that can damage the cells. Genetic injury can result in abnormal protein synthesis, impaired cell function, or inherited diseases.

Choice A reason: Bilirubin is a yellow pigment that is produced from the breakdown of hemoglobin in red blood cells. It is normally excreted by the liver into bile and then eliminated by the intestines. However, when there is an obstruction of bile flow in the liver due to cirrhosis, bilirubin accumulates in the blood and tissues, causing jaundice.

Choice B reason: Lipofuscin is a brown pigment that is derived from the oxidation of lipids and proteins in the cells. It is also known as "wear and tear" pigment because it accumulates with aging and reflects the cumulative damage to the cells.

Choice C reason: Steatosis is a condition where fat accumulates in the cytoplasm of hepatocytes (liver cells). It can be caused by various factors, such as alcohol abuse, obesity, diabetes, or malnutrition.

Choice D reason: Phenylketonuria is a genetic disorder where phenylalanine, an amino acid, cannot be converted into tyrosine, another amino acid, due to a deficiency of an enzyme called phenylalanine hydroxylase. This leads to a buildup of phenylalanine and its metabolites in the blood and tissues, causing brain damage and mental retardation.

Choice A reason: "An unregulated and passive process of cell death that causes inflammation and tissue damage." This is a description of necrosis, not apoptosis. Necrosis is an abnormal and harmful process that occurs when the cells are exposed to severe or persistent stressors that overwhelm their adaptive capacity. It involves the loss of cell membrane integrity, the release of cellular contents into the extracellular space, and the initiation of an inflammatory response that can damage the surrounding tissues.

Choice B reason: Apoptosis is also known as programmed cell death or cell suicide. It is a normal and beneficial process that maintains the balance between cell proliferation and cell death. It involves a series of biochemical events that lead to the activation of enzymes called caspases, which degrade the cell's DNA, proteins, and organelles. The cell then shrinks and forms membrane-bound fragments called apoptotic bodies, which are phagocytosed by macrophages or neighboring cells without causing inflammation or tissue damage.

Choice C reason: "A reversible process of cell injury that occurs when the stressor is mild or removed." This is a description of reversible cell injury, not apoptosis. Reversible cell injury is a process that occurs when the cells are exposed to mild or transient stressors that do not exceed their adaptive capacity. It involves changes in cell metabolism, function, and structure that can be restored if the stressor is removed or the cells are able to adapt.

Choice D reason: "An irreversible process of cell injury that occurs when the stressor is severe or persistent." This is a description of irreversible cell injury, not apoptosis. Irreversible cell injury is a process that occurs when the cells are exposed to severe or persistent stressors that exceed their adaptive capacity. It involves changes in cell metabolism, function, and structure that cannot be restored and lead to cell death by necrosis or apoptosis.

Choice A reason: Phenylketonuria (PKU) is a genetic disorder where phenylalanine, an amino acid, cannot be converted into tyrosine, another amino acid, due to a deficiency of an enzyme called phenylalanine hydroxylase. This leads to a buildup of phenylalanine and its metabolites in the blood and tissues, causing brain damage and mental retardation.

Choice B reason: Tyrosine is another amino acid that is normally produced from phenylalanine by the enzyme phenylalanine hydroxylase. However, in PKU, this enzyme is deficient, so tyrosine levels are low in the blood and tissues.

Choice C reason: Glucose is a simple sugar that is the main source of energy for the cells. It is not directly related to PKU, although some people with PKU may have low blood glucose levels due to dietary restrictions.

Choice D reason: Glycogen is a complex carbohydrate that is stored in the liver and muscles as a reserve source of energy. It is not directly related to PKU, although some people with PKU may have glycogen storage disease, which is a separate genetic disorder where glycogen cannot be broken down into glucose due to a deficiency of an enzyme called glucose-6-phosphatase.