Question

A 73 year old male presents to his physician complaining of cough and fatigue. His cough began 6 months ago and has steadily worsened, and is now associated with occasional expectoration of mucus streaked with bright red blood. Patient has also noted worsening dyspnea on exertion and a weight loss of 15 lbs. The patient has smoked a pack and a half of cigarettes every day for the past 60 years. Past medical history is significant for bipolar disorder treated with lithium. Physical exam shows unilateral localized wheezing on the left chest and clubbing of the distal extremities. Capillary refill is brisk. No skin tenting is observed. Chest X-ray is obtained, which shows a large mass at the left hilum. Labs show: Glucose 130, Na⁺ 125, K+ 4.0, Cl⁻ 91, HCO³⁻25, BUN 15, Creatinine 1.0; Plasma osmolality 270 (Normal: 282-295 mOsm/kg); Urine osmolality 650 mOsm/kg (Normal: 50 - 1400 mOsm/kg). Which of the following is the most likely mechanism for this patient's hyponatremia?

A. Increased oral intake of hypotonic fluids
B. Decreased oral intake of solutes
C. Impaired secretion of ADH in the posterior pituitary
D. Ectopic overproduction of vasopressin
E. Resistance to ADH action on the cortical and medullary collecting tubules

Answer 1

The patient's hyponatremia is most likely due to the ectopic overproduction of vasopressin by a lung mass, consistent with syndrome of inappropriate antidiuretic hormone secretion (SIADH).

Step-by-step explanation:

The most likely mechanism for the patient's hyponatremia is ectopic overproduction of vasopressin. This condition is also known as syndrome of inappropriate antidiuretic hormone secretion (SIADH). The patient's hyponatremia could be caused by a paraneoplastic syndrome, where the lung mass may be producing ADH or ADH-like substances independently, leading to water retention and dilute hyponatremia. The presented clinical features and lab values, including low plasma osmolality and relatively high urine osmolality, support this mechanism. Other options, such as increased oral intake of hypotonic fluids or decreased oral intake of solutes, are less compatible with the lab findings.