A 60-year-old man with small cell carcinoma of lung is rushed to emergency room in coma after suffering a clonic-tonic seizure. Patient’s temperature is 37degC , blood pressure 100/50 mm Hg and pulse is 88 per minute. Laboratory studies show a serum sodium of 103 mmol/L, normal serum levels of BUN & creatinine and a dilute but otherwise normal urine. A CT scan of the head is normal. Which of the following is the most likely cause of seizures in this patient?

**Core Concept**
The patient's presentation of seizures, hyponatremia, and normal urine osmolality in the context of small cell carcinoma of the lung suggests the diagnosis of the syndrome of inappropriate antidiuretic hormone secretion (SIADH), a condition characterized by excessive secretion of antidiuretic hormone (ADH) leading to water retention and subsequent hyponatremia.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of SIADH, where the ectopic production of ADH by small cell carcinomas leads to water retention, resulting in dilutional hyponatremia. In this patient, the normal urine osmolality suggests that the kidneys are able to dilute the urine in response to the excess ADH, leading to the development of seizures due to the severe hyponatremia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while cerebral metastasis is a possible cause of seizures in cancer patients, the normal CT scan of the head makes this option less likely. Additionally, cerebral metastasis would not typically cause hyponatremia.
**Option B:** This option is incorrect because while hypokalemia can cause seizures, the patient's potassium level is not mentioned, and the main issue in this case is the severe hyponatremia caused by SIADH.
**Option C:** This option is incorrect because while electrolyte imbalances can cause seizures, the patient's serum electrolyte levels are not provided, and the main issue in this case is the severe hyponatremia caused by SIADH.

**Clinical Pearl / High-Yield Fact**
Remember that small cell carcinomas of the lung are notorious for producing ectopic hormones, including ADH, leading to SIADH and hyponatremia. This is a classic exam trap, and students should always consider SIADH in the differential diagnosis of hyponatremia in cancer patients.

**Correct Answer:** C. Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

✓ Correct Answer: D. Syndrome of inappropriate ADH secretion