What test is performed on the patient prior to obtaining an ABG?

Choice A rationale

Diabetes insipidus is a condition characterized by the inability of the kidneys to conserve water, typically due to a deficiency in antidiuretic hormone (ADH) or kidney unresponsiveness to ADH. Post-neurosurgery, especially involving the pituitary or hypothalamus, ADH secretion can be impaired, leading to excessive urine output (polyuria), often exceeding 200 mL/hr, and dilute urine with a low specific gravity (normal range 1.005-1.030).

Choice B rationale

SIADH (Syndrome of Inappropriate Antidiuretic Hormone) is characterized by excessive ADH secretion, leading to water retention, hyponatremia, and concentrated urine. This condition would manifest as decreased urine output, not increased, and would be associated with a higher urine specific gravity (normal range 1.005-1.030) due to water reabsorption.

Choice C rationale

Acute Kidney Injury (AKI) involves a rapid decrease in kidney function, leading to the accumulation of waste products in the blood. While AKI can affect urine output, it typically results in oliguria (decreased urine output) or anuria (no urine output), rather than the profound polyuria observed in the patient.

Choice D rationale

Diabetes mellitus is a metabolic disorder characterized by high blood glucose levels due to insulin deficiency or resistance. While uncontrolled diabetes mellitus can cause polyuria due to osmotic diuresis from hyperglycemia, it would also present with other classic symptoms like polydipsia and polyphagia, and would be detected by elevated blood glucose levels (normal fasting plasma glucose <100 mg/dL).

Choice A rationale

Intubation into the cranial vault is anatomically impossible during standard endotracheal intubation. The tube is designed to pass through the oral or nasal pharynx, larynx, and vocal cords into the trachea. The cranial vault is a completely separate anatomical structure, superior to the airway, protected by the skull, and has no direct communication with the respiratory tract for tube misplacement.

Choice B rationale

While left mainstem bronchus intubation can occur, it is less common than right mainstem intubation. The left mainstem bronchus branches at a more acute angle from the trachea compared to the right, making it less likely for an endotracheal tube, which typically follows a straighter path, to inadvertently enter. This anatomical difference reduces the incidence of left-sided misplacement.

Choice C rationale

The right mainstem bronchus branches off the trachea at a less acute angle and is a more direct continuation of the tracheal lumen compared to the left mainstem bronchus. This anatomical alignment makes it the most common site for inadvertent endotracheal tube misplacement, leading to ventilation of only the right lung and potential collapse of the left lung.

Choice D rationale

Duodenal intubation is an anatomical impossibility for an endotracheal tube. The duodenum is part of the gastrointestinal tract, located inferior to the stomach, and has no direct anatomical connection with the respiratory system. Endotracheal intubation is specifically designed for airway management, not gastrointestinal access.