The mechanisms of action of calcitropic hormones are important for understanding the molecular basis of disease states related to calcium homeostasis.

A. Calcium binding (transport) protein (CaBP) enhances the movement of calcium from the brush border into the cytoplasm.
B. Receptors for calcitonin are present in the osteoclasts where they increase cAMP production.
C. Parathormone essentially works independently to mobilize bone mineral, and never in concert with vitamin D.
D. The major calctropic hormone, calcitriol, regulates intestinal calcium absorption.

Which one of the following options is INCORRECT in maintaining calcium homeostasis?

Correct option is C

EXPLANATION

A. Calcium binding (transport) protein (CaBP) enhances the movement of calcium from the brush border into the cytoplasm.
     This is correct – This is a true statement. CaBP facilitates intracellular calcium transport in intestinal epithelial cells, playing a role in calcium absorption.

B. Receptors for calcitonin are present in the osteoclasts where they increase cAMP production.
     This is correct – Calcitonin binds to receptors on osteoclasts, inhibiting bone resorption, and its receptor activation is associated with increased cAMP levels.

C. Parathormone essentially works independently to mobilize bone mineral, and never in concert with vitamin D.
     This is incorrect – Parathyroid hormone (PTH) acts in concert with vitamin D (specifically, calcitriol) to increase calcium absorption in the intestines and mobilize calcium from bone.

D. The major calcitropic hormone, calcitriol, regulates intestinal calcium absorption.
      This is correct – Calcitriol (the active form of vitamin D) increases calcium absorption in the intestine. This is a true statement.

So, the correct answer is  (c)  Only C