A patient develops central diabetes insipidus after a head injury.
Which mechanism explains this condition?
Decreased ADH production leading to inability to concentrate urine.
Increased ADH secretion causing water retention.
Increased aldosterone causing sodium retention.
Decreased oxytocin affecting renal perfusion.
The Correct Answer is A
Choice A rationale
Central diabetes insipidus results from a deficiency in the synthesis or release of antidiuretic hormone, also known as vasopressin, by the hypothalamus or posterior pituitary gland. ADH normally acts on the distal tubules and collecting ducts of the kidneys to promote water reabsorption. Without sufficient ADH, the kidneys cannot concentrate urine, leading to the excretion of large volumes of dilute urine, known as polyuria, and causing profound systemic dehydration and potential hypovolemic shock.
Choice B rationale
Increased secretion of antidiuretic hormone is the hallmark of the Syndrome of Inappropriate Antidiuretic Hormone, which is the opposite of diabetes insipidus. SIADH leads to excessive water retention, dilutional hyponatremia, and concentrated urine with high osmolality. In central diabetes insipidus, the problem is a lack of ADH, not an excess. Therefore, this mechanism would describe a fluid volume excess state rather than the fluid volume deficit and high serum osmolality seen in diabetes insipidus.
Choice C rationale
Aldosterone is a mineralocorticoid produced by the adrenal cortex that regulates sodium and potassium balance. While it does influence fluid volume by promoting sodium and water retention, it is not the primary hormone involved in the pathology of central diabetes insipidus. Diabetes insipidus specifically involves the water-regulating function of ADH. Increased aldosterone would typically lead to hypertension and hypokalemia rather than the massive water loss and dilute urine characteristic of a lack of vasopressin.
Choice D rationale
Oxytocin is a hormone released by the posterior pituitary that is primarily involved in uterine contractions during labor and milk ejection during breastfeeding. While it is structurally similar to ADH, it does not play a significant role in renal perfusion or the regulation of urinary concentration in the context of a head injury. A decrease in oxytocin would not explain the polyuria or thirst associated with diabetes insipidus, as its physiological targets are non-renal.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Choice A rationale
The Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is characterized by the excessive release of ADH, leading to water retention and dilutional hyponatremia. Serum sodium is typically less than 135 mEq/L, and serum osmolality is low, often less than 280 mOsm/kg. Despite the diluted blood, the kidneys continue to excrete concentrated urine with a high osmolality, usually greater than 100 mOsm/kg. This triad of findings is the classic presentation for SIADH in a clinical setting.
Choice B rationale
Hyperthyroidism involves an overproduction of thyroid hormones, which increases the metabolic rate and can lead to symptoms like tachycardia, weight loss, and heat intolerance. While it can affect fluid and electrolyte balance indirectly through increased perspiration or changes in renal blood flow, it does not typically present with the specific combination of hyponatremia, low serum osmolality, and inappropriately high urine osmolality that defines a primary disorder of water metabolism like SIADH.
Choice C rationale
Diabetes insipidus is the functional opposite of SIADH, characterized by a deficiency of ADH or a lack of response to it. This leads to the excretion of large volumes of very dilute urine, resulting in high serum osmolality and hypernatremia, where serum sodium exceeds 145 mEq/L. Patients with diabetes insipidus have a low urine osmolality, often less than 200 mOsm/kg, because they cannot concentrate their urine effectively, which contradicts the high urine osmolality seen in the patient.
Choice D rationale
Addison's disease is characterized by adrenal insufficiency, leading to low levels of cortisol and aldosterone. While this can cause hyponatremia and hyperkalemia due to the loss of sodium and retention of potassium in the kidneys, it is usually accompanied by hypovolemia and hypotension. The primary mechanism of hyponatremia in Addison's is sodium wasting, whereas in SIADH, the hyponatremia is dilutional. Addison's would typically present with a different clinical picture including high potassium.
Correct Answer is ["2"]
Explanation
31 Step 1 is 200 mg ÷ 100 mg/mL = 2 mL.
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