This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrine System, Subject name: Physiology
The function of which of the following is increased by an elevated parathyroid hormone concentration: 
Select and explain. 
A. Osteoclasts, B. Hepatic formation of 25-hydroxycholecalciferol, C. Phosphate reabsorptive pathways in the renal tubules, D. Action of osteoblasts only
A: Osteoclasts
Answer: A: Osteoclasts 
Explanation: PTH increases phosphate excretion in the urine and thereby depresses plasma phosphate levels. This phosphaturic action is due to a decrease in reabsorption of phosphate effects on NaPi-IIa in the proximal tubules PTH acts directly on bone to increase bone resorption and mobilize Ca2+. PTH also increases the formation of 1,25-dihydroxycholecalciferol, and this increases Ca2+ absorption from the intestine. On a longer time scale, PTH stimulates both osteoblasts and osteoclasts.