A patient is brought to the emergency room following a seizure. Electrolytes show SIADH. CT Scan demonstrates a lung mass. Which of the following forms of lung cancer is most likely to cause the described electrolyte imbalance?
**Core Concept**
Small cell lung carcinoma (SCLC) is a highly aggressive form of lung cancer that often presents with paraneoplastic syndromes, including the syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH leads to hyponatremia due to excessive release of antidiuretic hormone (ADH), causing water retention and dilutional hyponatremia.
**Why the Correct Answer is Right**
SCLC is known to produce ectopic ADH, leading to SIADH. This is due to the presence of ADH-producing neurosecretory granules in the tumor cells. The ectopic ADH stimulates the kidneys to reabsorb water, resulting in dilutional hyponatremia. This is a classic paraneoplastic syndrome associated with SCLC.
**Why Each Wrong Option is Incorrect**
**Option A:** Non-small cell lung carcinoma (NSCLC) is less likely to cause SIADH, although it can present with other paraneoplastic syndromes such as hypercalcemia due to parathyroid hormone-related protein (PTHrP).
**Option B:** Adenocarcinoma is the most common type of NSCLC and is less likely to cause SIADH compared to SCLC.
**Option C:** Squamous cell carcinoma can present with paraneoplastic syndromes, but SIADH is less common compared to SCLC.
**Clinical Pearl / High-Yield Fact**
SCLC is often associated with paraneoplastic syndromes, including SIADH, Cushing's syndrome, and Eaton-Lambert syndrome. These syndromes can occur before or after the diagnosis of lung cancer and can provide clues to the underlying diagnosis.
**Correct Answer:** C. Small cell lung carcinoma.