Vitamin D and Calcium Metabolism — MCQs
Calcium absorption is affected by-
Active form of Vitamin D is:
Secondary hyperparathyroidism due to Vit D deficiency shows :
Which of the following is seen in sarcoidosis
A patient is on a low calcium diet for 8 weeks. Which of the following increases to maintain serum calcium levels?
Which of the following organs are involved in the synthesis and activation of vitamin D?
Which of the following causes hypocalcemia:
Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?
A patient is found to have elevated levels of methylmalonic acid. This finding suggests a deficiency in which vitamin?
True statement about vitamin K is?
Vitamin D and Calcium Metabolism Indian Medical PG Practice Questions and MCQs
Question 1: Calcium absorption is affected by-
- A. Calcitriol
- B. PTH
- C. Proteins
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - **Calcitriol**, **parathyroid hormone (PTH)**, and **proteins** all play crucial roles in regulating calcium absorption and metabolism. - While calcitriol directly enhances intestinal calcium absorption, PTH indirectly influences it via calcitriol synthesis, and proteins are necessary for calcium transport and binding. *Calcitriol* - **Calcitriol** (1,25-dihydroxyvitamin D3) is the hormonally active form of vitamin D, which is essential for stimulating calcium absorption in the intestine. - It increases the synthesis of **calcium-binding proteins (calbindins)** in enterocytes, facilitating calcium uptake. *PTH* - **Parathyroid hormone (PTH)** primarily regulates calcium levels by stimulating its release from bone and increasing reabsorption in the kidneys. - It also indirectly enhances intestinal calcium absorption by stimulating the **renal conversion of 25-hydroxyvitamin D to calcitriol**. *Proteins* - Various **proteins** are involved in calcium transport and absorption, including calcium-binding proteins (e.g., calbindin) in the gut. - Dietary protein intake can also influence calcium balance; however, specific mechanisms regarding direct absorption are more complex and indirect compared to calcitriol.
Question 2: Active form of Vitamin D is:
- A. Calcifediol
- B. Paricalcitol
- C. Cholecalciferol
- D. Calcitriol (Correct Answer)
Explanation: ***Calcitriol*** - **Calcitriol** (1,25-dihydroxycholecalciferol) is the **biologically active form of vitamin D**. - It is produced primarily in the kidneys from calcifediol and is responsible for regulating **calcium and phosphate levels** in the body. *Calcifediol* - **Calcifediol** (25-hydroxyvitamin D) is the main circulating form of vitamin D, produced in the liver from cholecalciferol. - It is **not the active form**; rather, it is a precursor that needs further hydroxylation in the kidneys to become calcitriol. *Paricalcitol* - **Paricalcitol** is a **synthetic vitamin D analog** used therapeutically to prevent and treat secondary hyperparathyroidism. - It is not the naturally occurring active form of vitamin D in the body. *Cholecalciferol* - **Cholecalciferol** (vitamin D3) is the form of vitamin D synthesized in the skin upon exposure to sunlight or obtained from the diet. - It is a **precursor** to the active form and requires two hydroxylation steps (in the liver and kidneys) to become calcitriol.
Question 3: Secondary hyperparathyroidism due to Vit D deficiency shows :
- A. Hypocalcemia (Correct Answer)
- B. Hypophosphatemia
- C. Hypercalcemia
- D. Hyperphosphatemia
Explanation: ***Hypocalcemia*** - **Vitamin D deficiency** leads to decreased intestinal absorption of calcium, causing **hypocalcemia** [3]. - This persistent **low serum calcium** is the primary stimulus for the parathyroid glands to increase PTH secretion, leading to secondary hyperparathyroidism [1], [2]. *Hypophosphatemia* - While PTH typically promotes phosphate excretion in the kidneys leading to hypophosphatemia, in **secondary hyperparathyroidism due to vitamin D deficiency**, the effect on phosphate can be variable [3]. - The goal of increased PTH is to raise calcium, and maintaining some level of phosphate is necessary for bone health and proper calcium regulation. Early or mild deficiency may not show significant hypophosphatemia. *Hypercalcemia* - **Hypercalcemia** is a characteristic feature of **primary hyperparathyroidism**, where the parathyroid glands autonomously overproduce PTH [1]. - In secondary hyperparathyroidism (due to vitamin D deficiency), the PTH is elevated in response to **low calcium**, and sustained significant hypercalcemia is not expected; in fact, the underlying problem is **hypocalcemia** [1]. *Hyperphosphatemia* - **PTH** generally acts to lower serum phosphate levels by promoting its renal excretion [2]. - Therefore, **hyperphosphatemia** is not typically observed in secondary hyperparathyroidism; rather, a more common finding would be normal or low phosphate due to the elevated PTH [3].
Question 4: Which of the following is seen in sarcoidosis
- A. Decreased calcium levels in the blood (Hypocalcemia)
- B. Increased phosphate levels in the blood (Hyperphosphatemia)
- C. Increased calcium levels in the blood (Hypercalcemia) (Correct Answer)
- D. Decreased phosphate levels in the blood (Hypophosphatemia)
Explanation: ***Increased calcium levels in the blood (Hypercalcemia)*** - In sarcoidosis, activated macrophages in granulomas produce **1-alpha hydroxylase**, which converts **25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol)** [1]. - This increased calcitriol leads to enhanced intestinal **calcium absorption** and occasional bone resorption, resulting in **hypercalcemia** [1]. *Decreased calcium levels in the blood (Hypocalcemia)* - **Hypocalcemia** is not characteristic of sarcoidosis; the disease typically involves dysregulated vitamin D metabolism causing elevated, not decreased, calcium [1]. - Conditions like **hypoparathyroidism** or **vitamin D deficiency** (unrelated to the sarcoidosis pathogenesis) would cause hypocalcemia. *Increased phosphate levels in the blood (Hyperphosphatemia)* - **Hyperphosphatemia** is not a feature of sarcoidosis. Calcium and phosphate levels often have an inverse relationship, so with hypercalcemia, phosphate levels tend to be normal or slightly decreased due to suppression of parathyroid hormone. - Hyperphosphatemia is more commonly associated with **renal failure** or certain **endocrine disorders**. *Decreased phosphate levels in the blood (Hypophosphatemia)* - While **hypophosphatemia** can occur in conditions with high vitamin D activity (as increased active vitamin D can promote renal phosphate excretion), it is not the primary or most notable electrolyte disturbance specifically linked to sarcoidosis. - **Hypercalcemia** is the more consistently observed electrolyte abnormality in sarcoidosis related to ectopic vitamin D production [1].
Question 5: A patient is on a low calcium diet for 8 weeks. Which of the following increases to maintain serum calcium levels?
- A. Active 24,25 dihydroxy cholecalciferol
- B. PTH (Correct Answer)
- C. Serum phosphate level
- D. Calcitonin
Explanation: ***PTH*** - **Parathyroid hormone (PTH)** is the primary regulator of calcium homeostasis and the key hormone that **increases in response to hypocalcemia** (low serum calcium). - In a patient on a low calcium diet for 8 weeks, **PTH secretion increases** to maintain normal serum calcium levels. - PTH acts through three main mechanisms: increasing **bone resorption** (releasing calcium from bone), enhancing renal **calcium reabsorption** in the distal tubule, and stimulating the production of **active vitamin D (1,25-dihydroxycholecalciferol)** which increases intestinal calcium absorption. *Active 24,25 dihydroxy cholecalciferol* - **24,25-dihydroxycholecalciferol** is a relatively **inactive metabolite** of vitamin D and represents a pathway of vitamin D catabolism, not activation. - The **active form** of vitamin D that increases calcium absorption is **1,25-dihydroxycholecalciferol (calcitriol)**, whose production is stimulated by PTH. - This metabolite does **not increase** in response to hypocalcemia as a compensatory mechanism. *Serum phosphate level* - A low calcium diet would **not directly lead to an increase in serum phosphate levels**. - In fact, PTH (which increases in response to low calcium) typically causes a **decrease in serum phosphate** by promoting renal phosphate excretion (phosphaturic effect). - High phosphate levels can actually exacerbate hypocalcemia by forming insoluble calcium-phosphate complexes. *Calcitonin* - **Calcitonin** is released from the thyroid parafollicular cells (C cells) in response to **high serum calcium levels** (hypercalcemia). - It acts to **lower** calcium by inhibiting osteoclast activity and reducing renal calcium reabsorption. - In hypocalcemia (low calcium diet), calcitonin secretion would **decrease, not increase**, making this the opposite of what occurs to maintain calcium homeostasis.
Question 6: Which of the following organs are involved in the synthesis and activation of vitamin D?
- A. Liver and Skin
- B. Skin and Kidney
- C. Kidney and Liver
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - The **skin** synthesizes an inactive form of vitamin D (**cholecalciferol**) upon exposure to **UVB radiation**. - The **liver** performs the first hydroxylation step, converting **cholecalciferol** (vitamin D3) into **25-hydroxyvitamin D** (calcidiol). The **kidneys** then perform the final hydroxylation, converting **calcidiol** into the active form, **1,25-dihydroxyvitamin D** (calcitriol). *Liver and Skin* - While both the **liver** and **skin** play crucial roles in vitamin D metabolism, they do not encompass all necessary organs. - The **kidneys** are essential for the final activation step of vitamin D. *Skin and Kidney* - The **skin** synthesizes the precursor, and the **kidneys** perform the final activation step. - However, the **liver** is required for the initial hydroxylation of vitamin D. *Kidney and Liver* - The **kidney** is responsible for the final activation, and the **liver** for the initial hydroxylation. - This option misses the crucial role of the **skin** in the initial synthesis of vitamin D upon sun exposure.
Question 7: Which of the following causes hypocalcemia:
- A. 1, 25-dihydroxycholecalciferol
- B. Parathormone
- C. Thyroid hormones
- D. Calcitonin (Correct Answer)
Explanation: ***Calcitonin*** - **Calcitonin** is a hormone secreted by the **parafollicular cells (C cells)** of the thyroid gland. - It **lowers serum calcium levels** by **inhibiting osteoclast activity** (preventing bone resorption) and **increasing renal calcium excretion**. - This is the only hormone among the options that causes hypocalcemia. *1,25-dihydroxycholecalciferol* - This is the **active form of vitamin D** (calcitriol), which **increases serum calcium levels**. - It promotes intestinal calcium absorption, enhances bone resorption, and increases renal calcium reabsorption. - Deficiency of this hormone leads to hypocalcemia, but the hormone itself raises calcium. *Parathormone* - **Parathyroid hormone (PTH)** is the primary regulator that **increases serum calcium levels**. - It stimulates **osteoclast activity** (releasing calcium from bone), increases renal calcium reabsorption, and promotes synthesis of 1,25-dihydroxycholecalciferol. - PTH acts opposite to calcitonin in calcium homeostasis. *Thyroid hormones* - **Thyroxine (T4) and triiodothyronine (T3)** primarily regulate metabolism and have **no direct role in calcium homeostasis**. - While severe thyroid dysfunction can indirectly affect bone turnover, thyroid hormones do not directly cause hypocalcemia.
Question 8: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?
- A. Vitamin K
- B. Vitamin D (Correct Answer)
- C. Vitamin A
- D. Biotin
Explanation: ***Vitamin D*** - The skin synthesizes vitamin D (specifically **cholecalciferol**) upon exposure to **ultraviolet B (UVB) radiation** from sunlight. - This endogenous production can be sufficient to meet physiological needs under adequate sun exposure, making it conditionally non-essential in the diet. *Vitamin K* - While **intestinal bacteria synthesize some vitamin K (K2)**, it is generally not in sufficient quantities to meet all physiological needs, especially for blood clotting. - Dietary intake of **vitamin K1 (phylloquinone)** from leafy green vegetables is critical. *Vitamin A* - **Vitamin A (retinol)** is obtained primarily from the diet, either directly from animal sources or from carotenoid precursors (like **beta-carotene**) in plants. - The body cannot synthesize vitamin A de novo; it relies on dietary intake and conversion from precursors. *Biotin* - Although the **gut microbiota can synthesize biotin**, the amount produced is generally considered insufficient to meet the body's requirements. - Therefore, biotin is primarily obtained through dietary intake, functioning as a coenzyme in various metabolic reactions.
Question 9: A patient is found to have elevated levels of methylmalonic acid. This finding suggests a deficiency in which vitamin?
- A. Folate (important for DNA synthesis and red blood cell production).
- B. Vitamin D (essential for calcium and phosphate homeostasis).
- C. Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).
- D. Vitamin B12 (required for methylmalonic acid metabolism). (Correct Answer)
Explanation: ***Vitamin B12 (required for methylmalonic acid metabolism).*** - An elevated level of **methylmalonic acid** (MMA) is a sensitive and specific indicator of **vitamin B12 deficiency** because vitamin B12 is a coenzyme for the enzyme **methylmalonyl-CoA mutase**, which converts methylmalonyl-CoA to succinyl-CoA. - Without sufficient **vitamin B12**, MMA accumulates and can be measured in serum or urine, leading to **neurological symptoms** and **megaloblastic anemia**. *Vitamin B6 (involved in neurotransmitter synthesis and amino acid metabolism).* - **Vitamin B6** (pyridoxine) is a cofactor for enzymes involved in amino acid metabolism, neurotransmitter synthesis, and **heme synthesis**, but it does not directly metabolize methylmalonic acid. - Deficiency in vitamin B6 can cause **sideroblastic anemia**, **neuropathy**, and **dermatitis**, not elevated MMA. *Folate (important for DNA synthesis and red blood cell production).* - **Folate** (vitamin B9) is crucial for DNA synthesis, red blood cell maturation, and amino acid metabolism, often leading to **megaloblastic anemia** when deficient. - Although folate deficiency can also cause **macrocytic anemia**, it does not lead to an accumulation of **methylmalonic acid**, distinguishing it from vitamin B12 deficiency. *Vitamin D (essential for calcium and phosphate homeostasis).* - **Vitamin D** is primarily involved in **calcium and phosphate homeostasis**, bone health, and immune function. - Deficiency in vitamin D can cause **rickets** in children and **osteomalacia** in adults, but it has no direct role in the metabolism of **methylmalonic acid**.
Question 10: True statement about vitamin K is?
- A. Vitamin K is needed for action of clotting factor 8
- B. Vitamin K deficiency leads to DVT
- C. Vitamin K is water soluble
- D. Vitamin K affects bone health by activating proteins that bind calcium (Correct Answer)
Explanation: ***Vitamin K affects bone health by activating proteins that bind calcium*** - Vitamin K plays a crucial role in **osteocalcin activation**, a protein essential for integrating calcium into the bone matrix. - This activation process involves **gamma-carboxylation of glutamic acid residues** on osteocalcin, allowing it to bind calcium and strengthen bone. *Vitamin K is needed for action of clotting factor 8* - Vitamin K is essential for the activation of **clotting factors II, VII, IX, and X**, as well as proteins C and S, through gamma-carboxylation. - **Factor VIII** (antihemophilic factor A) is not directly dependent on vitamin K for its activation. *Vitamin K deficiency leads to DVT* - Vitamin K deficiency primarily leads to **impaired blood clotting** and an increased risk of bleeding, not DVT. - DVT (deep vein thrombosis) is typically associated with **hypercoagulable states**, venous stasis, or endothelial injury. *Vitamin K is water soluble* - Vitamin K is a **fat-soluble vitamin**, meaning it is absorbed with fats in the diet and stored in the body's fatty tissues and liver. - Water-soluble vitamins include **B vitamins and vitamin C**, which are not stored to the same extent and are excreted in urine.
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