Renal: MRCEM Primary questions with answers

Renal physiology covers how the kidney keeps the body's fluid, electrolytes and acid base balance steady. It is part of the physiology section of MRCEM Primary, which carries 60 of the 180 questions. The topic includes functional anatomy, autoregulation, glomerular filtration rate, creatinine clearance, sodium and potassium handling, urine concentration, the renin angiotensin aldosterone system and acid base disorders. It underpins the management of hyperkalaemia and acute kidney injury.

5 free questions below, each with the answer and the full explanation · 158 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Concentration and Dilution of Urine

   ADH acts mainly at which of the following sites in the renal nephron?

   1. AProximal convoluted tubule
   2. BDistal convoluted tubule
   3. CDescending limb of loop of Henle
   4. DThin ascending limb of loop of Henle
   5. EThick ascending limb of loop of Henle
   Show answer and explanation

   Answer: B. ADH acts via V2 receptors, inserting aquaporin-2 in the distal convoluted tubule and collecting duct.

   • A: Reabsorption here is not ADH-regulated.
   • C: Descending limb is water-permeable without ADH.
   • D: Thin ascending limb is impermeable to water.
   • E: Thick ascending limb is impermeable to water.

   Full explanation: Concentration and Dilution of Urine

   Concentration and dilution of urine 

   Regulation of plasma osmolarity 

   • It is accomplished by varying the amount of H2O excreted relative to the amount of solute excreted (i.e by varying urine osmolarity)

   Production of concentrated urine 

   • Also called hyperosmotic urine, in which urine osmolarity > Blood osmolarity
   • It is produced when circulating ADH levels are high (e.g., water deprivation, volume depletion, SIADH).

   Corticopapillary osmotic gradient – high ADH 

   • Is the gradient of osmolarity from the cortex (300mOsm/L) to the papilla (1200mOsm/L) and is composed primarily of NaCl and urea.
   • Is established by countercurrent multiplication and urea recycling.
   • Is maintained by countercurrent exchange in the vasa recta.

     Countercurrent multiplication in the LOH 

   • Depends upon NaCl reabsorption in the thick ascending limb and countercurrent flow in the descending and ascending limbs of LOH.
   • It is augmented by ADH, which stimulates NaCl reabsorption in the thick ascending limb. Therefore, the presence of ADH increases the size of corticopapillary osmotic gradient.

    Urea recycling  

   • Urea recycling from inner medullary collecting ducts into the medullary interstitial fluid is also augmented by ADH (by stimulating UT1 transporter) 

   Vasa recta  

   • Are the capillaries that supply the LOH. They maintain the corticopapillary gradient by serving as osmotic exchanger.
   • Vasa recta blood equilibrates osmotically with interstitial fluid of medulla and papilla.

   Proximal tubule – high ADH  

   • The osmolarity of glomerular filtrate is identical to that of plasma (300mOsm/L).
   • Two third of filtered H2O is reabsorbed isosmotically (with Na, Cl, H2O, glucose, amino acids and so forth) in the proximal tubule.
   • TF/Posm = 1.0 throughout the proximal tubule because water is reabsorbed isosmotically with solute. 

   Thick ascending limb of LOH – high ADH  

   • It is called diluting segment.
   • Reabsorbs NaCl by the NA-K-2Cl cotransporter, reabsorption of Na+ is further enhanced by ADH.
   • Impermeable to H2O, therefore H2O is not reabsorbed with NaCl, and the tubular fluid becomes diluted.
   • The fluid that leaves the thick ascending limb has an osmolarity of 100 mOsm/L and TF/Posm < 1.0 as a result of dilution process.

   Early distal tubule – high ADH  

   • It is called cortical diluting segment.
   • Like the thick ascending limb, early distal tubule reabsorbs NaCl but is impermeable to water. Consequently, tubular fluid is further diluted.

   Late distal tubule – high ADH

   • ADH increases the H2O permeability in the principal cells of late distal tubule.
   • H2O is reabsorbed from the tubular fluid until the osmolarity of tubular fluid equals that of surrounding interstitial fluid in renal cortex (300mOsm/L)
   • TF/Posm = 1.0 at the end of the distal tubule because osmotic equilibration occurs in the presence of ADH.

   Collecting ducts – high ADH  

   • As in the late distal tubule, ADH increases the H2O permeability of the principal cells of the collecting ducts.
   • As tubular fluid flows through the collecting ducts, it passes through the corticopapillary gradient (regions of increasingly higher osmolarity), which was previously established by countercurrent multiplication and urea recycling.
   • H2O is reabsorbed from the collecting ducts until the osmolarity of tubular fluid equals that of the surrounding fluid.
   • The osmolarity of the final urine equals that at the bend of LOH and tip of papilla (1200mOsm/L)
   • TF/Posm= >1.0 because osmotic equilibration occurs with the corticopapillary gradient in the presence of ADH.

   Production of dilute urine   

   • Is called hypoosmotic urine, in which urine osmolarity < blood osmolarity.
   • Produced when circulating levels of ADH are low (e.g., water intake, central diabetes insipidus) or when ADH is ineffective (nephrogenic diabetes insipidus)

   Corticopapillary osmotic gradient – no ADH  

   • Is smaller than in the presence of ADH because ADH stimulates both countercurrent multiplication and urea recycling

   Proximal tubule – no ADH  

   • Two third of filtered water is reabsorbed isosmotically.
   • TF/Posm = 1.0 throughout the proximal tubule

   Thick ascending limb of LOH – no ADH  

   • NaCl is reabsorbed without water and tubular fluid becomes (although not quite as dilute as in the presence of ADH.
   • TF/Posm = < 1.0

     Early distal tubule – no ADH 

   • NaCl is reabsorbed without water and tubular fluid is further diluted.
   • TF/Posm = < 1.0 

   Late distal tubule – no ADH  

   • In the absence of ADH, the cells of late distal tubule and collecting ducts are impermeable to water. 
   • Thus, even though the tubular fluid flows through the corticopapillary osmotic gradient, osmotic equilibration does not occur.
   • The osmolarity of final urine will be diluted with an osmolarity as low as 50mOsm/L.
   • TF/Posm = < 1.0

   Free water clearance (CH2O)  

   • It is used to estimate the ability to concentrate or dilute the urine.
   • Free water or solute free water is produced in the diluting segment of kidney (i.e. thick ascending limb and early distal tubule), where NaCl is reabsorbed, and free H2O is left behind in the tubular fluid.
   • In absence of ADH, this solute free water is excreted and CH2O is positive.
   • In presence of ADH, this solute free water is not excreted but is reabsorbed by the late distal tubule and collecting ducts and CH2O is negative.

   Urine that is isosmotic to plasma

   • CH2O is zero 
   • Produced during a treatment with a loop diuretic, which inhibits NaCl reabsorption in the thick ascending limb, inhibiting both dilution in the thick ascending limb and production of the corticopapillary osmotic gradient. Therefore, the urine can’t be diluted during high water intake or concentrated during water deprivation.

   Urine that is hypoosmotic to plasma (low ADH) CH2O is positive. 

   • Produced with high water intake (in which ADH released from the posterior pituitary is suppressed), central diabetes insipidus or nephrogenic diabetes insipidus.

   Urine that is hyperosmotic to plasma (high ADH)  

   • CH2O is negative.
   • It is produced in water deprivation or SIADH.

    

2. Question 2 · Renal Regulation of Urea, Phosphate, Calcium and Magnessium

   You are reviewing a patient with infected arteriovenous fistula who will undergo hemofiltration tomorrow. Which of the following best describes the Transport of molecules across the membrane that occurs in haemofiltration?

   1. ASimple diffusion
   2. BOsmosis
   3. CFacilitated diffusion
   4. DActive transport
   5. EUltrafiltration
   Show answer and explanation

   Answer: E. Haemofiltration removes fluid and solutes by ultrafiltration across the membrane under a pressure gradient.

   • A: Diffusion is the main mechanism of haemodialysis, not haemofiltration.
   • B: Water movement down an osmotic gradient is not the main driver.
   • C: Carrier-mediated process of cells, not a filtration membrane.
   • D: Energy-dependent cellular transport, not membrane filtration.

   Full explanation

   • Ultrafiltration refers to the process of movement of fluids through a membrane caused by a pressure gradient
   • In dialysis, fluid is removed by ultrafiltration using the dialysis membrane. The pressure on the dialysate side is lower so water moves from the blood (place of higher pressure) to the dialysate (place of lower pressure). This is how the hemodialysis treatment removes fluid.

3. Question 3 · Nacl Regulation

   A male aged 50 presents to ED with headache and chest pain; he is currently taking captopril and Hydrochlorothiazide for hypertension. Thiazide diuretics act at which of the following sites in the nephron?

   1. AEarly part of distal convoluted tubule
   2. BCollecting duct
   3. CLate part of distal convoluted tubule
   4. DThin ascending limb of loop of Henle
   5. EThick ascending limb of loop of Henle
   Show answer and explanation

   Answer: A. Thiazides inhibit the Na+/Cl- cotransporter in the early distal tubule.

   • B: Site of potassium-sparing diuretics.
   • C: Thiazides act on the early, not late, segment.
   • D: Not the thiazide site.
   • E: Site of loop diuretic action.

   Full explanation: Nacl Regulation

   NaCl Regulation 

   • Tubular fluid (TF) is urine at any point along the nephron. 
   • Plasma (P) is systemic plasma. It is considered to be constant. 

   TF/P ratio: Compares the conc of a substance in tubular fluid at any point along the nephron with the conc in plasma. 

   • If TF/P= 1.0, then either there has been no reabsorption of the substance or reabsorption of substance has been exactly proportional to the reabsorption of water.  
   • If TF/P < 1.0, then reabsorption of substance has been greater than the reabsorption of water and the conc in tubular fluid is less than that in plasma. 
   • If TF/P>1.0, then either reabsorption of the substance has been less than the reabsorption of water or there has been secretion of the substance. 

   Na reabsorption along the nephron: 

   Na is freely filtered across the glomerular capillaries; therefore, the Na in the TF of bowman space equals that in plasma i.e. TF/PNa = 1.0 

   Proximal tubule:  

   • Is the site of glomerulotubular balance. 
   • The process is isosmotic. The reabsorption of Na and H2O in PT is exactly proportional. 

   Early proximal tubule: 

   • Reabsorbs 2/3rd, or 67% of the filtered Na and H2O more than any other part of the nephron. 
   • Reabsorbs Na and H2O with HCO3, glucose, amino acids, phosphate and lactate. 
   • Na is reabsorbed by cotransport with glucose, amino acids, phosphate and lactate. These cotransport processes account for the reabsorption of all the filtered glucose and amino acids. 
   • Na is also reabsorbed by counter transport via Na-H exchange, which is linked directly to the reabsorption of filtered HCO3 
   • Carbonic anhydrase inhibitors (acetazolamide) are diuretics that act in the early pct by inhibiting the reabsorption of filtered HCO3. 
   • Reabsorbs 67% of the filtered K along with Na and H2O 
   • Reabsorbs 50% of filtered urea. 
   • Reabsorbs 85% of filtered phosphate. 

   Late proximal tubule:  

   • Filtered glucose, amino acids and HCO3 have already been completely removed from the tubular fluid by reabsorption in early pct. 
   • In the late pct, Na is reabsorbed with Cl (Cotransport). 
   • Effects of ECF volume on PT reabsorption: 
   • ECF volume contraction increases PT reabsorption. 
   • ECF volume expansion decreases PT reabsorption. 

   Thick ascending limb of loop of Henle: 

   • Reabsorbs 25% of the filtered Na 
   • Reabsorbs 20% of filtered K 
   • Contains a Na-K-2Cl cotransporter in the luminal membrane. 
   • Is the site of action of the loop diuretics (furosemide, ethacrynic acid, bumetanide), which inhibit the Na-K-2Cl cotransporter. 
   • Is impermeable to H2O. Thus, NaCl is reabsorbed without water. As a result, tubular fluid [Na] and tubular fluid osmolarity decreases to less than their conc in plasma. This segment therefore is called the diluting segment. 
   • It has a lumen positive potential difference. Although the Na-K-2Cl cotransport appears to be electroneutral, some K diffuses back into the lumen, making the lumen electrically positive.  

   Distal tubule and collecting duct: 

   •   Together reabsorb 8% of filtered Na. 

   Early DCT: 

   • Reabsorbs NaCl by a Na-Cl cotransporter. 
   • Is the site of action of thiazide diuretics 
   • Is impermeable to water, as thick ascending limb. Thus, reabsorption of NaCl occurs without water, which further dilutes the tubular fluid, therefore called cortical diluting segment. 
   • Thiazide diuretics cause hypocalciuria, in a way that they block the Na-Cl cotransporter and decrease the Na reabsorption and also Na-K ATPase on the basolateral membrane maintains a low intracellular conc: of Na, which causes the Na from interstitial fluid/peritubular capillary blood to move into the cell in exchange with Ca through Na-Ca exchanger/counter transport thereby increasing the reabsorption of Ca into blood. 
   • PTH increases the activity of Na-Ca exchanger and increases the reabsorption of Ca. 

   Late DCT: Have two cell types. 

   Principal cells Reabsorbs Na and H2O 

   • Secretes K 
   • Aldosterone increases Na reabsorption and increases K secretion. Like other steroid hormones, the action of aldosterone takes several hours to develop because new protein synthesis of Na channels (ENaC) is required. About 2% of overall Na reabsorption is affected by aldosterone. 
   • ADH increases H2O permeability by directing the insertion of H2O channels in the luminal membrane. In the absence of ADH, the principal cells are virtually impermeable to water. 
   • K- sparing diuretics (spironolactone, triamterene, amiloride) decreases K secretion.  

   Alpha intercalated cells: 

   • Secretes H+ by an H-adenosine triphosphatase (H-ATPase), which is stimulated by aldosterone. 
   • Reabsorbs K+ by an H-K-ATPase 

4. Question 4 · Renal

   A male aged 55 years, a known case of HTN and CKD, presents with easy fatigability and pale skin. Lab values reveal HB: 8 and decreased level of hematocrit. Peripheral film shows normocytic normochromic anemia. What is the most likely cause?

   1. ADecrease erythropoietin
   2. BHemolytic anemia
   3. CIncrease erythropoietin
   4. DSideroblastic anemia
   5. EAnemia of chronic disease
   Show answer and explanation

   Answer: A. CKD reduces renal erythropoietin synthesis, causing a normocytic normochromic anaemia.

   • B: Haemolysis is not the main mechanism in CKD anaemia.
   • C: Erythropoietin falls in CKD; raised levels would stimulate red cell production.
   • D: Sideroblastic anaemia is typically microcytic, not the usual cause in CKD.
   • E: Chronic disease contributes, but erythropoietin deficiency is the main cause.

   Full explanation

   In patients with chronic kidney disease, normochromic normocytic anemia mainly develops from decreased renal synthesis of erythropoietin.

    

5. Question 5 · Functional Anatomy of Kidney

   A diabetic patient aged 60 presents to the ED with increased thirst, frequent urination, and fatigue. Lab results reveal elevated blood glucose levels and the presence of glucose in the patient's urine. Which part of the kidney reabsorbs the greatest amount of glucose?

   1. ACollecting duct
   2. BLoop of Henle
   3. CDistal convoluted tubule
   4. D: Renal papilla
   5. EProximal convoluted tubule
   Show answer and explanation

   Answer: E. Most glucose is reabsorbed in the proximal convoluted tubule.

   • A: Not a site of glucose reabsorption.
   • B: Not the main site of glucose reabsorption.
   • C: Little or no glucose reabsorption here.
   • D: Not a site of glucose reabsorption.

   Full explanation: Functional Anatomy of Kidney

   Anatomy of Kidney 

   The kidneys are two bean-shaped organs, located on either side of the spine, in the retroperitoneal space. The left kidney is situated a little higher than the right one, because of the liver on the right side of the abdominal cavity, above the right kidney. The kidneys have a medial dimple called the renal hilum, which is the entry and exit point for structures that supply or drain the kidneys such as the nerves, ureters, vessels, and lymphatics.  

   Parts of the Kidney 

   • The cortex is the outer renal tissue located under the renal capsule. It appears lighter in colour compared to the other parts of the kidney. The renal corpuscles, as well as the proximal and distal convoluted tubules are found here. It reaches into the inner renal tissue or medulla, splitting it into triangular parts called renal pyramids. 
   • The renal pyramids contain the loops of Henle of each renal tubule, as well as the collecting ducts. Each of the renal pyramids opens at the apex, or renal papilla. Here the urine formed inside the pyramids is collected through a structure called the minor calyx. Several of the minor calices join to form a major calyx. Urine moves through the major calices to reach the renal pelvis. The pelvis is a funnel-shaped collecting structure formed by the junction of all the major calices. It carries urine from the whole kidney to the ureter, which begins where the pelvis ends, at the pelvi-ureteral junction.  

   Nephrons 

   • The nephrons are the functional units of the kidneys, and number about 1.3 million per kidney.  
   • There are two types of nephrons, cortical nephrons (85% of nephrons) and juxtamedullary nephrons (15% of nephrons) 
   • A nephron has two main parts – tubules (proximal convoluted tubule, the loop of Henle and the distal convoluted tubule, collecting duct) and renal corpuscle (Bowman’s capsule and glomerulus). 
   • Cortical nephrons have their renal corpuscles in the outer cortex and relatively short loops of Henle (LOH). 
   • Juxtamedullary nephrons have their corpuscles in the inner third of cortex near corticomedullary junction with long LOH extending into renal medulla. 
   • Glomerulus is formed by the invagination of ball of capillaries into the bowman’s capsule. 

   Main functions of nephron  

   • Glomerulus – filtration 
   • Proximal tubule – Reabsorption of solutes 
   • Loop of Henle – urine concentration 
   • Distal tubule and collecting duct – Acid base balance and H2O homeostasis. 

   Functions of kidney 

   • Excretes waste products: Urea, uric acid, creatinine. 
   • Water and electrolyte balance 
   • Acid/base balance 
   • Metabolism of proteins including PTH and calcitonin 
   • Secretes the hormone erythropoietin and the enzyme, renin into the circulation, synthesis of prostaglandins 
   • Hydroxylates 25-hydroxy-Vit D to form the active form of Vitamin D (1,25 dihydroxy-Vit D) 

   Juxtaglomerular apparatus 

    The juxtaglomerular apparatus (JGA) is located between the afferent arteriole and the returning distal convoluted tubule of the same nephron. It is responsible for regulating both intrarenal (tubuloglomerular feedback) and extrarenal (renin-angiotensin-aldosterone) mechanisms necessary to maintain both renal and entire body volume status. 

   The three components of the JGA are the following: 

   • The juxtaglomerular cells (granular cells) of the afferent arteriole, synthesize and store renin, which is secreted in response to specific stimuli (e.g., low blood flow, decreased NaCl delivery). The juxtaglomerular cells could be considered the “effector arm” of the renin-angiotensin-aldosterone axis. 
   •  The macula densa, a region of the distal convoluted tubule characterized by columnar tubular epithelial cells which are more densely packed than in other regions of the nephron. The macula densa can be considered the “sensory arm” of the renin-angiotensin-aldosterone axis in that these are the cells which sense and respond to changes in tubular Na Cl concentration. They are also involved in the mechanism of tubuloglomerular feedback. 
   • mesangial cells, which form connections via actin and microtubules which allow for selective vasoconstriction/vasodilation of the renal afferent and efferent arterioles with mesangial cell contraction. 

   Glomerular filtration barrier 

   A membrane that selectively allows the passage of molecules based on size and charge. Plasma is selectively filtered through glomerular capillary wall into the bowman’s capsule (Ultrafiltrate = fluid filtered into bowman’ space) 

   This barrier has three major components. 

   • 1: Glomerular capillary endothelium: It has fenestrations which allows plasma components with a molecular weight of <7000 Da to pass freely. These fenestrations actually do not restrict the movement of water and proteins or large molecules but instead prevent the filtration of blood cells (e.g. RBCs).
   • Fenestrated endothelium has magnitude greater than proteins; in addition, the wall is covered with negatively charged compounds. 
   • 2: Glomerular basement membrane: Made up of matrix of extracellular negatively charged proteins and other compounds, help restrict the movement of negatively charged molecules across the basement membrane. e.g., Albumin
   •  Main site of ultrafiltrate
   • The basement membrane consists of 3 layers: These layers help to limit the filtration of intermediate and large sized solutes e.g., Albumin.
   • 3: Epithelial cell layer of podocytes: Next to the bowman’s space; the podocytes have foot processes directly contacting the basement membrane. 
   • Regular gaps between foot processes (pedicles) are called filtration slits, across these slits a protein network forms slit pores in the epithelial lining which prevent the passage of larger molecules through this final layer. 
   • Podocytes also exhibit phagocytic and contractile activity like that of mesangial cells.