A 45-year-old woman presents to the emergency department with perioral paresthesias, carpopedal spasm, and a positive Trousseau sign. An ECG is obtained and shows the finding marked **A** in the diagram. Which of the following is the most likely underlying diagnosis?

Explanation

## Why Hypocalcemia is right The clinical triad of neuromuscular hyperexcitability (perioral paresthesias, carpopedal spasm, positive Trousseau sign) combined with the ECG finding marked **A** — a prolonged QT interval (QTc 560 ms) with a characteristic **stretched ST segment** while the T wave morphology remains relatively preserved — is pathognomonic for hypocalcemia. Per Harrison 21e Ch 84 and Ch 405, hypocalcemia (defined as total serum calcium < 8.5 mg/dL or ionized calcium < 4.5 mg/dL) causes prolonged QT specifically due to prolongation of the ST segment, distinguishing it from other electrolyte disturbances. The Trousseau sign (carpal spasm on blood pressure cuff inflation) is a highly specific clinical sign of hypocalcemia-induced neuromuscular hyperexcitability. This patient requires immediate IV calcium gluconate and correction of any concurrent hypomagnesemia. ## Why each distractor is wrong - **Hypokalemia**: While hypokalemia also prolongs the QT interval, the ECG pattern is distinctly different — hypokalemia causes T-wave flattening or inversion, prominent U waves, and ST depression. The characteristic "stretched ST segment" with preserved T-wave morphology seen in **A** is not typical of hypokalemia. Additionally, hypokalemia presents with muscle weakness and fatigue, not the hyperexcitability and Trousseau sign seen here. - **Hypomagnesemia**: Although hypomagnesemia can prolong QT and impair PTH secretion (worsening hypocalcemia), it does not independently produce the clinical triad of Trousseau sign and carpopedal spasm. Hypomagnesemia must be corrected concurrently with calcium, but the primary diagnosis driving the neuromuscular signs and characteristic ECG pattern is hypocalcemia. - **Hypophosphatemia**: Hypophosphatemia does not cause QT prolongation or the neuromuscular hyperexcitability signs (Trousseau, carpopedal spasm) described in this patient. It is typically asymptomatic unless severe, and its ECG manifestations are minimal. It may be a secondary finding in vitamin D deficiency but is not the primary diagnosis. **High-Yield:** Hypocalcemia = Trousseau sign + prolonged QT with **stretched ST segment** (not T-wave flattening); always correct magnesium concurrently; treat with IV calcium gluconate 10-20 mL of 10% solution over 10-20 minutes. [cite: Harrison 21e Ch 84 (Disorders of Calcium, Phosphate, and Magnesium Metabolism) + Ch 405 (Electrocardiography)]