Renal Filtration and Electrolyte Balance Practice Questions
20 free Renal Filtration and Electrolyte Balance practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following best describes what is filtered at the glomerulus during normal glomerular filtration?
- A Water and small solutes, but not proteins
- B Plasma including proteins and cells
- C Cellular elements but no proteins
- D Only nitrogenous wastes such as urea
Correct answer: Water and small solutes, but not proteins
Glomerular filtration produces a plasma-like filtrate that excludes most plasma proteins and cells; water and small solutes (ions, small molecules) are filtered. Larger proteins and blood cells are normally retained in the capillaries.
Which parameter is defined as the volume of plasma filtered into Bowman's capsule per minute and is widely used to assess kidney function?
- A Renal blood flow (RBF)
- B Renal plasma flow (RPF)
- C Glomerular filtration rate (GFR)
- D Filtration fraction (FF)
Correct answer: Glomerular filtration rate (GFR)
GFR represents the volume of plasma filtered across all glomeruli into Bowman's space per unit time, and is the standard measure of kidney filtration performance.
What is the approximate normal value of filtration fraction (FF) in a healthy adult?
- A 5%
- B 10%
- C 20%
- D 50%
Correct answer: 20%
The filtration fraction (FF = GFR / RPF) in normal physiology is around 20 %, meaning about one-fifth of the plasma flowing through the kidneys is filtered into nephrons.
Which segment of the nephron reabsorbs the largest proportion of the filtered sodium load under normal conditions?
- A Proximal convoluted tubule (PCT)
- B Thick ascending limb of loop of Henle
- C Distal convoluted tubule
- D Collecting duct
Correct answer: Proximal convoluted tubule (PCT)
Approximately 65% of filtered sodium is reabsorbed in the proximal tubule, driven by basolateral Na⁺/K⁺-ATPase and apical NHE3 (and other cotransporters).
Which transporter in the thick ascending limb of the loop of Henle is most critical for reabsorption of sodium, potassium, and chloride?
- A ENaC
- B Na⁺/glucose cotransporter (SGLT2)
- C Na-K-2Cl cotransporter (NKCC2)
- D Na⁺/H⁺ exchanger (NHE3)
Correct answer: Na-K-2Cl cotransporter (NKCC2)
The NKCC2 cotransporter in the thick ascending limb reabsorbs Na⁺, K⁺, and Cl⁻, and is essential for sodium reabsorption in this segment.
Which hormone increases sodium reabsorption and potassium secretion in the distal nephron, thereby affecting electrolyte and volume balance?
- A Antidiuretic hormone (ADH)
- B Aldosterone
- C Atrial natriuretic peptide (ANP)
- D Parathyroid hormone (PTH)
Correct answer: Aldosterone
Aldosterone acts on distal tubule and collecting duct principal cells, upregulating Na⁺/K⁺-ATPase and ENaC channels, increasing sodium and water reabsorption and promoting potassium secretion.
In conditions of dietary potassium depletion, how does the kidney primarily reduce urinary K⁺ excretion?
- A Increase electrogenic sodium reabsorption in distal nephron to drive K⁺ secretion
- B Decrease K⁺ reabsorption in proximal tubule
- C Decrease distal sodium delivery and reduce K⁺ channel abundance in principal cells
- D Increase K⁺ filtration at glomerulus
Correct answer: Decrease distal sodium delivery and reduce K⁺ channel abundance in principal cells
With low dietary potassium, distal sodium delivery is reduced and K⁺-secreting channels are downregulated in the distal nephron, decreasing K⁺ excretion to conserve potassium.
Which of the following best describes how the kidneys contribute to acid–base balance?
- A By filtering plasma proteins into the urine
- B By reabsorbing bicarbonate and secreting H⁺ ions
- C By generating carbon dioxide in tubular cells
- D By secreting urea into the filtrate
Correct answer: By reabsorbing bicarbonate and secreting H⁺ ions
The kidneys maintain acid–base homeostasis by reabsorbing filtered bicarbonate (HCO₃⁻) and secreting H⁺ ions into the urine; this adjusts blood pH over time.
If afferent arteriolar constriction occurs (with no change in efferent arteriole), what is the expected effect on GFR and RPF (renal plasma flow)?
- A GFR increases, RPF decreases
- B GFR decreases, RPF decreases
- C GFR unchanged, RPF decreases
- D GFR increases, RPF increases
Correct answer: GFR decreases, RPF decreases
Constriction of the afferent arteriole reduces renal plasma flow and decreases glomerular hydrostatic pressure, thereby lowering both RPF and GFR.
What happens to the sodium reabsorption in the proximal tubule when basolateral Na⁺/K⁺-ATPase is inhibited?
- A Sodium reabsorption increases
- B Sodium reabsorption decreases
- C No effect on sodium reabsorption
- D Sodium secretion occurs
Correct answer: Sodium reabsorption decreases
Inhibition of Na⁺/K⁺-ATPase disrupts the sodium gradient that drives sodium reabsorption across the proximal tubule, thereby reducing sodium (and consequently water) reabsorption.
Which of the following is a major mechanism by which the kidneys regulate extracellular fluid volume and blood pressure via sodium handling?
- A Changing the glomerular filtration barrier permeability
- B Altering glomerular capillary oncotic pressure
- C Hormonal modulation of sodium and water reabsorption
- D Adjusting afferent arteriole diameter only
Correct answer: Hormonal modulation of sodium and water reabsorption
The kidneys regulate ECF volume and blood pressure by adjusting sodium (and thus water) reabsorption in response to hormones (e.g., aldosterone, ADH, RAAS), thereby modulating volume and osmolarity.
A patient with decreased distal sodium delivery to the collecting duct would most likely experience which electrolyte change?
- A Decreased potassium excretion (leading to hyperkalemia)
- B Increased potassium excretion (leading to hypokalemia)
- C Increased calcium excretion
- D Increased phosphate excretion
Correct answer: Decreased potassium excretion (leading to hyperkalemia)
Reduced distal sodium delivery decreases the driving force for potassium secretion in the distal nephron principal cells, leading to reduced K⁺ excretion and potential hyperkalemia.
Which of the following statements about potassium handling by the kidney is correct under normal dietary potassium intake?
- A All filtered potassium is reabsorbed, so none appears in urine
- B Freely filtered, mostly reabsorbed proximally, then fine-tuned distally
- C Potassium is mostly secreted in the proximal tubule
- D Potassium is neither filtered nor reabsorbed by the nephron
Correct answer: Freely filtered, mostly reabsorbed proximally, then fine-tuned distally
Potassium is freely filtered at the glomerulus; a large portion is reabsorbed (e.g., proximal tubule), and the distal nephron/collecting duct adjust K⁺ excretion to match dietary intake and maintain balance.
Which principle explains why water follows sodium reabsorption along much of the nephron, helping to concentrate urine and regulate volume?
- A Active transport of water by aquaporins only
- B Osmosis following solute and sodium reabsorption
- C Hydrostatic pressure within Bowman's capsule space
- D Adjustment of the glomerular filtration fraction
Correct answer: Osmosis following solute and sodium reabsorption
As sodium (and other solutes) is reabsorbed from the filtrate, osmotic gradients are generated that draw water passively across the tubular epithelium, aiding water reabsorption and volume regulation.
Which nephron segment is impermeable to water but actively reabsorbs ions, contributing to the counter-current multiplier system and concentrating the medullary interstitium?
- A Proximal tubule
- B Thin descending limb of loop of Henle
- C Thick ascending limb of loop of Henle
- D Collecting duct
Correct answer: Thick ascending limb of loop of Henle
The thick ascending limb is impermeable to water but reabsorbs Na⁺, K⁺, and Cl⁻ via NKCC2; by doing so, it dilutes tubular fluid and contributes to the hyperosmolar medullary gradient essential for urine concentration.
Which of the following would most likely decrease glomerular filtration rate (GFR)?
- A Constriction of the efferent arteriole
- B Dilation of the afferent arteriole
- C Decrease in plasma oncotic (colloid) pressure
- D Increase in Bowman's capsule hydrostatic pressure
Correct answer: Increase in Bowman's capsule hydrostatic pressure
An increase in hydrostatic pressure within Bowman's capsule opposes filtration pressure, thereby reducing net filtration and decreasing GFR.
Under conditions of volume depletion, activation of the renin–angiotensin–aldosterone system (RAAS) will have which effect on the nephron?
- A Decrease sodium reabsorption in the proximal tubule
- B Increase excretion of both sodium and water
- C Reabsorb more sodium and water; excrete potassium
- D Promote water excretion independent of sodium
Correct answer: Reabsorb more sodium and water; excrete potassium
RAAS activation leads to aldosterone release, which increases sodium (and water) reabsorption and enhances potassium excretion — restoring fluid volume at the cost of increased K⁺ loss.
Which function is NOT primarily a role of the kidneys in renal physiology and electrolyte balance?
- A Regulation of plasma osmolarity and volume
- B Excretion of metabolic waste and toxins
- C Generation of red blood cells directly
- D Maintenance of acid–base and electrolyte homeostasis
Correct answer: Generation of red blood cells directly
While the kidneys produce erythropoietin (which stimulates red blood cell production), they do not generate red blood cells themselves; that occurs in the bone marrow. The kidneys’ primary roles include filtration, excretion, fluid/electrolyte balance, acid–base regulation, and volume control.
Which statement best describes the importance of hormonal control (e.g., ADH, aldosterone) over simple filtration in maintaining long-term electrolyte and fluid homeostasis?
- A Filtration alone suffices for electrolyte balance regardless of intake
- B Hormones adjust tubular reabsorption and secretion per diet and volume
- C Hormones influence balance only during renal failure
- D Hormonal effects only alter glomerular filtration rate
Correct answer: Hormones adjust tubular reabsorption and secretion per diet and volume
While filtration constantly produces a large filtrate, hormonal regulation adjusts reabsorption and secretion in the tubules based on volume, electrolyte intake and physiologic needs, thus maintaining homeostasis more precisely.
Which of the following correctly explains how the kidney conserves potassium when dietary potassium is low?
- A By increasing distal sodium delivery to enhance potassium secretion
- B By reducing sodium reabsorption throughout the nephron
- C By reducing distal flow and downregulating K⁺ secretory channels
- D By increasing glomerular filtration of potassium
Correct answer: By reducing distal flow and downregulating K⁺ secretory channels
In potassium depletion, the kidney reduces distal tubular flow and downregulates luminal K⁺ channels in the distal nephron, thereby reducing K⁺ secretion and conserving body potassium.