A patient presents with muscle weakness, ileus, and an ECG showing flat T waves with prominent U waves. Which physiological mechanism best explains the ECG changes in severe hypokalaemia?

• A Increased resting membrane potential making cells hyperexcitable
• B Hyperpolarization of cardiomyocytes slowing phase 3 repolarization
• C Reduced Na-K ATPase activity shortening the action potential
• D Decreased intracellular K+ causing delayed ventricular repolarization ✓

Explanation

In hypokalaemia, decreased intracellular K+ reduces the electrochemical gradient for K+ efflux during phases 2 and 3 of the action potential. This delays repolarization of ventricular cells, prolonging the QT interval and producing the characteristic flat T waves and prominent U waves on ECG. The U wave represents late repolarization of Purkinje fibres or papillary muscles, exaggerated when K+ is low.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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