This question requires integrating knowledge from two different units of the Class 11 Biology curriculum: Locomotion and Movement and Chemical Coordination and Integration. According to the sources, tetany is a disorder characterised by rapid muscle spasms caused by low Ca2+ levels in body fluids . The parathyroid hormone (PTH) acts as a hypercalcemic hormone that increases blood Ca2+ levels . Consequently, a deficiency in PTH leads to hypocalcemia, which results in tetany . Other options are incorrect: gigantism results from excess Growth Hormone , cretinism results from thyroid hormone deficiency during pregnancy , and diabetes insipidus is caused by a deficiency of ADH (vasopressin) . Mastering these NCERT-based clinical associations is essential as approximately 95% of NEET Biology questions are derived directly from these primary textbooks .