Which statement correctly describes an antigen?
A cell that engulfs pathogens.
A mediator of inflammation.
A substance that triggers an immune response.
A protein produced by B cells.
The Correct Answer is C
Choice A rationale
A cell that engulfs pathogens is known as a phagocyte, such as a macrophage or a neutrophil. These are specialized white blood cells that identify, ingest, and destroy foreign invaders through a process called phagocytosis. While an antigen may be present on the surface of the pathogen that the phagocyte consumes, the cell itself is an effector of the innate immune system and is not synonymous with the molecular structure of an antigen.
Choice B rationale
Mediators of inflammation include substances like histamines, leukotrienes, and prostaglandins that are released by mast cells and other leukocytes to initiate the inflammatory response. These chemicals increase vascular permeability and blood flow to the site of injury or infection. While antigens can trigger the release of these mediators during an allergic or immune response, they are distinct molecular entities that serve as the identification markers rather than the signaling chemicals themselves.
Choice C rationale
An antigen is any substance, typically a protein or polysaccharide, that the immune system recognizes as foreign and that subsequently triggers an immune response. Antigens can be found on the surfaces of viruses, bacteria, fungi, and pollen. Once detected, they stimulate the production of specific antibodies or the activation of T cells. The specificity of the immune response depends on the unique molecular shape of the antigen which fits with a specific receptor.
Choice D rationale
Proteins produced by B cells in response to an antigen are called antibodies or immunoglobulins. These Y-shaped proteins circulate in the blood and bind specifically to the antigen that triggered their production, marking it for destruction by other immune cells. The antigen is the target or the stimulus, whereas the antibody is the defensive protein produced by the body to neutralize or eliminate that specific threat from the internal environment.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is C
Explanation
Choice C rationale
Normal negative feedback loops involve the end-product (cortisol) inhibiting the production of the stimulating hormone (ACTH). In a functioning system, high cortisol levels would signal the pituitary to stop producing ACTH. However, a pituitary adenoma is an autonomous tumor that does not respond to these inhibitory signals. Therefore, the persistence of high ACTH levels despite the presence of high circulating cortisol (normal: 5 to 23 mcg/dL) clearly demonstrates that the negative feedback mechanism is disrupted and no longer regulating hormone secretion.
Choice A rationale
Fluctuating cortisol levels with normal ACTH levels suggest that the pituitary-adrenal axis is still somewhat responsive to physiological rhythms or that the primary issue may not be at the pituitary level. In the case of an ACTH-secreting adenoma, one would expect both values to be consistently abnormal or poorly regulated. Normal ACTH ranges (typically 10 to 60 pg/mL) in the presence of fluctuating cortisol do not provide the definitive evidence of feedback loop failure that a non-suppressible ACTH value provides.
Choice B rationale
This finding would actually demonstrate that the negative feedback loop is still intact. If elevated cortisol levels were successful in suppressing ACTH secretion, it would indicate that the pituitary gland is still capable of sensing and responding to the concentrations of adrenal hormones in the blood. In Cushing disease caused by a pituitary adenoma, the hallmark is that the tumor continues to secrete ACTH even when cortisol levels are excessively high, proving the loss of normal regulatory control.
Choice D rationale
Decreased cortisol levels despite elevated ACTH would typically indicate primary adrenal insufficiency, such as Addison disease, rather than a pituitary adenoma. In this scenario, the pituitary is functioning correctly by increasing ACTH to stimulate the failing adrenal glands, but the adrenals are unable to produce cortisol. This represents a functioning feedback loop attempting to maintain homeostasis, rather than the disruption of feedback seen when a pituitary tumor ignores high hormone levels and continues to overproduce ACTH. .
Correct Answer is D
Explanation
Choice A rationale
Restricting fluids would be contraindicated in a patient with adrenal insufficiency experiencing hypotension. These patients suffer from a profound deficit of mineralocorticoids, specifically aldosterone, which leads to massive urinary loss of sodium and water. Normal serum sodium is 135 to 145 mEq/L. The resulting hypovolemia requires aggressive fluid resuscitation with isotonic saline to restore circulating volume and blood pressure. Fluid restriction would worsen the cardiovascular collapse and exacerbate the Addisonian crisis.
Choice B rationale
Providing potassium supplements is dangerous because hyperkalemia is already present in adrenal insufficiency. Normal serum potassium is 3.5 to 5.0 mEq/L. Without aldosterone, the kidneys cannot effectively excrete potassium in exchange for sodium. Adding more potassium would increase the risk of life threatening cardiac arrhythmias or cardiac arrest. The clinical goal is to lower the potassium levels through volume expansion and corticosteroid replacement rather than increasing the total body potassium load.
Choice C rationale
Administering insulin is a temporary measure used to shift potassium from the extracellular fluid into the intracellular compartment. While it addresses the hyperkalemia, it does not treat the underlying cause of the crisis, which is a lack of cortisol and aldosterone. Furthermore, patients in adrenal crisis are often hypoglycemic due to glucocorticoid deficiency. Giving insulin without dextrose could cause fatal hypoglycemia. It is not the primary priority compared to replacing the missing hormones.
Choice D rationale
Administering intravenous hydrocortisone is the priority intervention because it provides both glucocorticoid and mineralocorticoid activity. This replacement therapy addresses the underlying hormonal deficit, helping to restore vascular tone, increase blood glucose levels, and promote the renal retention of sodium and excretion of potassium. By correcting the hormonal imbalance, hydrocortisone stabilizes the hemodynamics and electrolyte disturbances. This intervention is essential to reverse the life threatening systemic effects of an acute adrenal or Addisonian crisis.
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