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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{YD}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Physiology
|MainCategory=Physiology
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|MainCategory=Physiology
|MainCategory=Physiology
|SubCategory=Renal
|SubCategory=Renal
|MainCategory=Physiology
|MainCategory=Physiology
|MainCategory=Physiology
|MainCategory=Physiology
|MainCategory=Physiology
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|MainCategory=Physiology
|MainCategory=Physiology
|SubCategory=Renal
|SubCategory=Renal
|Prompt=A researcher is studying the renal handling of calcium along the nephron using rats. In his experiment, he notices that after filtration, more than 90% of calcium is reabsorbed by passive and active mechanisms. Additionally, he notes that calcium reabsorption is dependent on the hormonal effects of parathyroid hormone (PTH). Using the illustration below, which segment of the nephron is responsible for the PTH-mediated reabsorption of calcium?
|Prompt=A researcher is studying renal calcium dynamics along the length of a nephron. He conducts an experiment by sampling filtrate samples at various levels along the renal tubules. He notices that after filtration, more than 90% of calcium is reabsorbed by passive and active mechanisms. Additionally, he notes that calcium reabsorption is dependent on the hormonal effects of parathyroid hormone (PTH). Using the illustration below, which segment of the nephron is responsible for the PTH-mediated reabsorption of calcium?


[[Image:WBR0511.png|500px]]
[[Image:WBR0511.png|500px]]
|Explanation=Calcium homeostasis is important for osteogenesis and resorption of bone. Calcium is maintained in physiological concentrations under the control of parathyroid hormone (PTH). Low serum calcium promote the secretion of PTH into the circulation. PTH allows the resorption of calcium and phosphate in the bones. However, it causes increased GI absorption and renal reabsorption of calcium but decreased phosphate reabsorption in the kidney. As such, PTH generally increases the serum calcium concentrations. Calcium itself acts to inhibit the secretion of PTH in a negative-feedback mechanism. In the kidneys, PTH inhibits the sodium/phosphate co-transporter in the proximal convoluted tubules to promote phosphate excretion. In converse, it increases the calcium/sodium exchanger in the distal convoluted tubule to increase calcium reabsorption and thus increase the serum concentrations of calcium.
|Explanation=Calcium homeostasis is essential for muscular and neural function as well as osteogenesis. It is maintained in physiological concentrations under the control of parathyroid hormone (PTH). Low serum calcium promotes the secretion of PTH into the circulation allowing 3 main processes to occur: (1) increased calcium and phosphate release from bone, (2) increased calcium reabsorption/phosphate excretion in the renal tubules, and (3) increased calcium and phosphate reabsorption from the small intestine secondary to the PTH mediated increase in 1,25(OH)Vitamin D3. Accordingly, PTH increases the serum concentrations of calcium. The increase in calcium above a set threshold acts as an inhibitor of PTH secretion by a negative-feedback loop. In the kidneys, PTH inhibits the sodium/phosphate co-transporter in the proximal convoluted tubules to promote phosphate excretion (decrease its reabsorption). In converse, it increases the calcium/sodium exchanger in the distal convoluted tubule to increase calcium reabsorption and thus increase the serum concentrations of calcium.
 
Educational Objective:
PTH increases the calcium/sodium exchanger in the distal convoluted tubules to increase the reabsorption of calcium.
|AnswerA=A
|AnswerA=A
|AnswerAExp="A" corresponds to the proximal convoluted tubules. This is not the location where PTH acts to increase the reabsorption of calcium. However, this is the location where the sodium/phosphate co-transporter is inhibited to further excrete phosphate and decrease serum concentrations of phosphate.
|AnswerAExp="A" corresponds to the proximal convoluted tubules. This is not the location where PTH acts to increase the reabsorption of calcium. However, this is the location where the sodium/phosphate co-transporter is inhibited to further excrete phosphate and decrease serum concentrations of phosphate.
|AnswerB=B
|AnswerB=B
|AnswerBExp="B" corresponds to the descending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.  
|AnswerBExp="B" corresponds to the descending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.
|AnswerC=C
|AnswerC=C
|AnswerCExp="C" corresponds to the ascending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.  
|AnswerCExp="C" corresponds to the ascending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.
|AnswerD=D
|AnswerD=D
|AnswerDExp="D" corresponds to the distal convoluted tubule. This is the location where PTH-mediated calcium reabsorption occurs.
|AnswerDExp="D" corresponds to the distal convoluted tubule. This is the location where PTH-mediated calcium reabsorption occurs.
|AnswerE=E
|AnswerE=E
|AnswerEExp="E" corresponds to the collecting ducts. This is not the location where PTH acts to increase the reabsorption of calcium.  
|AnswerEExp="E" corresponds to the collecting ducts. This is not the location where PTH acts to increase the reabsorption of calcium.
|RightAnswer=C
|EducationalObjectives=PTH increases the calcium/sodium exchanger in the distal convoluted tubules to increase the reabsorption of calcium.
|WBRKeyword=calcium, serum, concentration, renal, handling, kidney, hormone, hormonal, PTH, parathyroid, phosphate, distal, convoluted, tubule, proximal, reabsorption
|References=Molina P. Endocrine Physiology, Fourth Edition. Chapter 5. Parathyroid Gland and Ca<sup>2+</sup> and PO<sub>4</sub><sup>-</sup> Regulation. McGraw Hill Professional; 2013.<br>
|Approved=No
First Aid 2014 page 314
|RightAnswer=D
|WBRKeyword=Calcium, Renal calcium handling, Kidney, PTH, Parathyroid hormone, Phosphate, Distal convoluted tubule, Proximal convoluted tubule, Electrolyte homeostasis
|Approved=Yes
}}
}}

Latest revision as of 00:49, 28 October 2020
Author [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Physiology
Sub Category SubCategory::Renal
Prompt [[Prompt::A researcher is studying renal calcium dynamics along the length of a nephron. He conducts an experiment by sampling filtrate samples at various levels along the renal tubules. He notices that after filtration, more than 90% of calcium is reabsorbed by passive and active mechanisms. Additionally, he notes that calcium reabsorption is dependent on the hormonal effects of parathyroid hormone (PTH). Using the illustration below, which segment of the nephron is responsible for the PTH-mediated reabsorption of calcium?

]]

Answer A AnswerA::A
Answer A Explanation [[AnswerAExp::"A" corresponds to the proximal convoluted tubules. This is not the location where PTH acts to increase the reabsorption of calcium. However, this is the location where the sodium/phosphate co-transporter is inhibited to further excrete phosphate and decrease serum concentrations of phosphate.]]
Answer B AnswerB::B
Answer B Explanation AnswerBExp::"B" corresponds to the descending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.
Answer C AnswerC::C
Answer C Explanation AnswerCExp::"C" corresponds to the ascending loop of Henle. This is not the location where PTH acts to increase the reabsorption of calcium.
Answer D AnswerD::D
Answer D Explanation AnswerDExp::"D" corresponds to the distal convoluted tubule. This is the location where PTH-mediated calcium reabsorption occurs.
Answer E AnswerE::E
Answer E Explanation AnswerEExp::"E" corresponds to the collecting ducts. This is not the location where PTH acts to increase the reabsorption of calcium.
Right Answer RightAnswer::D
Explanation [[Explanation::Calcium homeostasis is essential for muscular and neural function as well as osteogenesis. It is maintained in physiological concentrations under the control of parathyroid hormone (PTH). Low serum calcium promotes the secretion of PTH into the circulation allowing 3 main processes to occur: (1) increased calcium and phosphate release from bone, (2) increased calcium reabsorption/phosphate excretion in the renal tubules, and (3) increased calcium and phosphate reabsorption from the small intestine secondary to the PTH mediated increase in 1,25(OH)Vitamin D3. Accordingly, PTH increases the serum concentrations of calcium. The increase in calcium above a set threshold acts as an inhibitor of PTH secretion by a negative-feedback loop. In the kidneys, PTH inhibits the sodium/phosphate co-transporter in the proximal convoluted tubules to promote phosphate excretion (decrease its reabsorption). In converse, it increases the calcium/sodium exchanger in the distal convoluted tubule to increase calcium reabsorption and thus increase the serum concentrations of calcium.

Educational Objective: PTH increases the calcium/sodium exchanger in the distal convoluted tubules to increase the reabsorption of calcium.
References: Molina P. Endocrine Physiology, Fourth Edition. Chapter 5. Parathyroid Gland and Ca2+ and PO4- Regulation. McGraw Hill Professional; 2013.
First Aid 2014 page 314]]

Approved Approved::Yes
Keyword WBRKeyword::Calcium, WBRKeyword::Renal calcium handling, WBRKeyword::Kidney, WBRKeyword::PTH, WBRKeyword::Parathyroid hormone, WBRKeyword::Phosphate, WBRKeyword::Distal convoluted tubule, WBRKeyword::Proximal convoluted tubule, WBRKeyword::Electrolyte homeostasis
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