An anxious 22-year-old woman hyperventilates during a panic attack and develops carpopedal spasm. The immediate biochemical event responsible for the neuromuscular irritability is:

• A Direct effect of low carbon dioxide on skeletal muscle membrane channels
• B Rise in total serum calcium concentration driving muscle excitation
• C Fall in ionised calcium caused by increased binding of calcium to plasma proteins as pH rises ✓
• D Respiratory acidosis lowering ionised magnesium

Explanation

Hyperventilation lowers carbon dioxide and raises pH. Alkalosis increases the negative charge on plasma proteins, promoting calcium binding and decreasing the ionised fraction even though total calcium is unchanged. Reduced ionised calcium lowers the threshold for nerve and muscle depolarisation, producing tetany with Chvostek and Trousseau signs. Option A ignores the well-defined humoral pathway examined in standard texts.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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