Question

blood $ca^{2+}$ deficiency stimulates ________ secretion, which leads to ________
multiple choice

calcitonin; more urinary phosphate reabsorption

calcitriol; more urinary phosphate excretion

parathyroid hormone; increased osteoclast activity

growth hormone; increased osteoblast activity

thyroid hormone; less urinary calcium excretion

Explanation:

• Calcitonin: Secreted when blood \(Ca^{2+}\) is high, not low. It decreases osteoclast activity and increases urinary \(Ca^{2+}\) excretion. So the option with calcitonin is wrong.
• Calcitriol: Mainly involved in \(Ca^{2+}\) absorption from the intestine. Its relation to urinary phosphate excretion as the main effect in response to low blood \(Ca^{2+}\) is not the key here.
• Parathyroid hormone (PTH): Secreted when blood \(Ca^{2+}\) is low. PTH acts on osteoclasts (via osteoblast - osteoclast coupling). It stimulates osteoclast precursor cells to differentiate into active osteoclasts (increases osteoclast activity), which break down bone matrix and release \(Ca^{2+}\) into the blood.
• Growth hormone: Primarily for growth, cell reproduction, and regeneration. Not directly related to the immediate response to low blood \(Ca^{2+}\) in terms of bone cell activity as described.
• Thyroid hormone: Involved in metabolism, growth, and development. Not the hormone secreted in response to low blood \(Ca^{2+}\) for the described bone - related effect.

Answer:

parathyroid hormone; increased osteoclast activity