Which of the following is not normally found in plasma?

Choice A rationale

Glycogen is the storage form of glucose, primarily found within cells, particularly liver and muscle cells, as a readily accessible energy reserve. It is a polysaccharide, and its presence in plasma is not typical because glucose is the primary circulating carbohydrate, maintained at a narrow homeostatic range (normal fasting plasma glucose: 70-99 mg/dL or 3.9-5.5 mmol/L) for cellular energy.

Choice B rationale

Fibrinogen is a soluble plasma protein synthesized by the liver, crucial for hemostasis. Upon activation by thrombin, it polymerizes into insoluble fibrin strands, forming the structural meshwork of a blood clot, essential for stopping bleeding. Its normal concentration in plasma is approximately 200-400 mg/dL.

Choice C rationale

Glucose is a monosaccharide, the body's main source of energy, and is transported in the plasma to cells throughout the body for metabolic processes. Its concentration is tightly regulated by hormones like insulin and glucagon to ensure adequate cellular fuel while preventing hyperglycemia or hypoglycemia.

Choice D rationale

Urea is a metabolic waste product derived from the breakdown of proteins, specifically from the deamination of amino acids in the liver. It is transported in the plasma to the kidneys for excretion in urine, playing a vital role in nitrogenous waste elimination. Normal blood urea nitrogen (BUN) levels range from 7-20 mg/dL.

Choice A rationale

Innate immunity is the body's first line of defense, providing immediate, non-specific protection without prior exposure. Cytotoxicity refers to the ability of certain immune cells, like natural killer cells, to kill target cells, but it is not a distinct form of immunity that "remembers" pathogens.

Choice B rationale

Adaptive immunity is characterized by its specificity and immunological memory, meaning it learns and remembers specific pathogens. Innate immunity, however, does not possess memory; its responses are always the same regardless of previous exposure.

Choice C rationale

A natural killer cell is a component of innate immunity, providing non-specific cytotoxicity against infected or cancerous cells without prior sensitization. A macrophage is a phagocytic cell involved in both innate and adaptive immunity, but neither specifically represents the "memory" aspect of adaptive immunity in this context.

Choice D rationale

Innate immunity provides immediate, non-specific defense mechanisms against pathogens, but it lacks immunological memory, meaning it responds identically to repeated exposures. In contrast, adaptive immunity, also known as acquired immunity, develops memory cells that allow for a faster, stronger, and more specific response upon subsequent encounters with the same pathogen.