Substance X increases serum calcium and urine phosphate excretion. Its mechanism of action most closely resembles that of __________.
Correct Answer: Recombinant parathyroid hormone
Mini-Explanation: Like PTH, Substance X increases bone resorption and renal calcium reabsorption (raising serum calcium) while inhibiting renal phosphate reabsorption (increasing phosphaturia).
Distractors:
1,25-dihydroxyvitamin D analog: Increases intestinal absorption of calcium and phosphate, but does not cause prominent phosphaturia.
FGF23 inhibitor: Decreases renal phosphate excretion (causes phosphate retention).
Pyrophosphate analog & RANK-L inhibitor: These are anti-resorptive drugs (e.g., bisphosphonates, denosumab) that lower serum calcium.
Hook: Intermittent recombinant PTH (teriparatide) is an ANABOLIC agent for osteoporosis, unlike the chronic catabolic effect of endogenous PTH.
In decompensated heart failure, decreased renal perfusion triggers RAAS activation, leading to __________.
Correct Answer: Increased proximal tubular sodium reabsorption
Mini-Explanation: Angiotensin II from RAAS activation directly stimulates the Na+/H+ exchanger in the proximal tubule, promoting sodium (and water) retention.
Distractors:
Decreased distal tubule sodium reabsorption: This would cause natriuresis, which is counterproductive in heart failure.
Decreased proximal tubule urea reabsorption: Urea reabsorption passively follows sodium and water, so it is increased, not decreased.
Decreased renal venous pressure: Renal venous pressure is actually increased due to elevated central venous pressure from the failing heart.
Increased collecting duct free water excretion: ADH release in heart failure increases free water reabsorption, not excretion.
Increased renal blood flow: Renal blood flow is decreased, triggering the entire cycle.
Hook: The BUN/Cr ratio is often >20:1 in pre-renal states like heart failure due to increased urea reabsorption.
The rate of solute removal during hemodialysis is increased by a higher concentration gradient, higher temperature, and a larger membrane __________.
Correct Answer: Surface area
Mini-Explanation: A larger surface area provides more space for diffusion to occur, directly increasing the number of molecules that can cross the membrane per unit time.
Distractors:
Adding lithium to the dialysate: Decreases the concentration gradient, slowing removal.
Decreasing dialysis solution temperature: Slows molecular movement, decreasing diffusion.
Decreasing the membrane pore size: Restricts solute passage, especially for larger molecules.
Increasing thickness of the membrane: Increases the diffusion distance, slowing the rate.
Hook: This follows Fick’s law of diffusion.
A drug that decreases the renal tubular __________ of another drug will reduce its urinary excretion rate.
Correct Answer: Secretion
Mini-Explanation: Tubular secretion is an active process. A competing drug (Drug Y) can inhibit transporters, reducing the secretion and thus the overall excretion of the first drug (Drug X).
Distractors:
Decreased renal tubular reabsorption: This would increase the drug’s excretion rate.
Displacement from plasma proteins: This increases the free drug available for filtration, increasing excretion.
Increased glomerular filtration: This directly increases the drug’s filtration and excretion rate.
Hook: This is a classic example of drug-drug competition for organic anion/cation transporters in the proximal tubule.
In hypovolemia, renal plasma flow (RPF) decreases more than glomerular filtration rate (GFR), leading to a(n) __________ in filtration fraction.
Correct Answer: Increase
Mini-Explanation: Efferent arteriolar constriction (via angiotensin II) maintains glomerular hydrostatic pressure, so GFR falls less than RPF. Since FF = GFR/RPF, the fraction increases.
Distractors:
RPF ↑, GFR ↑, FF ↑: RPF and GFR do not increase in hypovolemia.
RPF ↓, GFR ↓, FF ↓: This occurs if both decrease proportionally, but compensatory mechanisms prevent this.
RPF ↓, GFR ↑, FF ↑: GFR does not increase above normal in hypovolemia.
RPF ↓, GFR ↓↓, FF ↓: This pattern is seen with a loss of autoregulation (e.g., in severe shock), not in the initial compensatory phase.
Hook: Angiotensin II’s selective efferent constriction is the key mechanism preserving GFR and increasing FF in pre-renal states.
In the proximal tubule, the tubular fluid/plasma concentration ratio for creatinine __________, while the ratio for glucose __________.
Correct Answers: increases; decreases
Mini-Explanation: Creatinine is secreted into the tubule, increasing its concentration. Glucose is actively and completely reabsorbed, decreasing its concentration.
Distractors:
Urea: Ratio increases (due to water reabsorption, not secretion).
Sodium/Potassium: Ratio remains ~1 (reabsorbed with water proportionally).
Bicarbonate: Ratio decreases (actively reabsorbed).
Hook: The slope of the concentration ratio graph directly indicates secretion (upward) vs. reabsorption (downward).
In hypovolemic shock, compensatory mechanisms lead to increased reabsorption of sodium, water, and __________.
Correct Answer: Urea
Mini-Explanation: ADH increases water and urea reabsorption in the collecting duct to help maintain the medullary concentration gradient and conserve volume.
Distractors:
Decreased chloride reabsorption: Chloride follows sodium reabsorption, so it is increased.
Decreased sodium reabsorption: RAAS and aldosterone increase sodium reabsorption.
Decreased urine osmolality: Urine is concentrated (high osmolality) due to ADH.
Increased renal blood flow: Renal blood flow is decreased, triggering the compensatory mechanisms.
Increased tubular hydrostatic pressure: This pressure is decreased due to low renal perfusion.
Hook: The high BUN/Cr ratio (>20:1) in pre-renal azotemia is partly due to this increased urea reabsorption.
In the absence of ADH (diabetes insipidus), tubular fluid is most concentrated at the __________ of the loop of Henle.
Correct Answer: Bottom (or tip)
Mini-Explanation: The descending limb is water-permeable. As it dips into the hyperosmotic medulla, water is drawn out, concentrating the tubular fluid to its maximum at the loop’s bend.
Distractors:
Proximal Tubule: Fluid remains iso-osmotic with plasma.
Early Distal Tubule: Fluid is hypotonic due to solute reabsorption without water in the thick ascending limb.
Collecting Duct: In the absence of ADH, this segment is impermeable to water, so fluid remains very dilute.
Hook: The countercurrent multiplier creates the medullary gradient; the descending limb is where tubular fluid concentration increases, and the ascending limb is where it decreases.
A high BUN/creatinine ratio (>20:1), low urine sodium, and a bland urinalysis are characteristic of __________.
Correct Answer: Pre-renal azotemia
Mini-Explanation: This pattern indicates reduced renal perfusion (e.g., from dehydration), triggering intact tubules to maximally reabsorb sodium and urea, without causing intrinsic damage (hence the bland UA).
Distractors:
Chronic pyelonephritis: Urinalysis typically shows WBCs and casts.
Interstitial nephritis: Urinalysis shows WBCs, WBC casts, and often eosinophils.
Renal artery stenosis: Causes hypertension and is a chronic cause of pre-renal physiology.
Tubular necrosis: Causes granular/muddy brown casts and a high FENa (>2%).
Urinary tract obstruction: May show crystals or hematuria; BUN/Cr ratio is usually <15:1.
Hook: The “pre-renal” state means the problem is before the kidney (hypoperfusion), so the kidney itself is functioning correctly, just conservatively.
The earliest change in renal tubular cells during sublethal ischemia is the __________, which disrupts sodium reabsorption.
Correct Answer: Loss of epithelial cell polarity
Mini-Explanation: ATP depletion disrupts the cytoskeleton, causing Na+/K+ ATPase pumps to relocate from the basolateral to the apical membrane, destroying the transcellular sodium gradient.
Distractors:
Activation of cytoplasmic caspases: This is a feature of apoptosis, which occurs later with more severe, lethal injury.
Damage to the tubular basement membrane: This is a late consequence of severe necrosis.
Necrosis and sloughing of epithelial cells: This is a later event following the initial loss of polarity and adhesion.
Hook: This loss of polarity leads to increased sodium delivery to the macula densa, causing vasoconstriction and perpetuating the injury (tubuloglomerular feedback).
In unilateral renal artery stenosis, the non-stenotic (contralateral) kidney responds to systemic hypertension with increased __________.
Correct Answer: Sodium excretion
Mini-Explanation: The non-stenotic kidney is exposed to high systemic pressure and RAAS mediators, leading to a pressure natriuresis to lower blood volume.
Distractors:
Decreased inferior vena cava aldosterone level: Aldosterone levels are increased due to RAAS activation.
Decreased systemic vascular resistance: Systemic vascular resistance is increased due to angiotensin II.
Increased glomerular filtration in the right kidney: GFR is decreased in the stenotic kidney.
Increased renin production in the left kidney: Renin production is suppressed in the non-stenotic kidney.
Hook: This is the body’s attempt to correct the hypertension, but it is overridden by the persistent renin release from the stenotic kidney.
A patient with hypocalcemia (positive Chvostek’s sign) and a history of a thyroid disorder most likely has a history of recent __________.
Correct Answer: Thyroid surgery
Mini-Explanation: Surgical removal of or damage to the parathyroid glands during thyroidectomy is the most common cause of acute hypoparathyroidism and hypocalcemia.
Distractors:
Excessive vitamin D intake: Causes hypercalcemia.
Frequent antacid use: (If calcium-containing) causes hypercalcemia.
New prescription for chlorothiazide: Causes mild hypercalcemia.
Nonadherence with levothyroxine: Causes hypothyroidism, which does not typically cause hypocalcemia.
Hook: The classic triad: post-thyroid surgery + neuromuscular irritability (Chvostek/Trousseau) + hypocalcemia.
The bone pathology in primary hyperparathyroidism is characterized by __________, most prominent in cortical bone.
Correct Answer: Subperiosteal resorption with cortical thinning
Mini-Explanation: Excess PTH increases osteoclast activity, leading to bone resorption that begins just beneath the periosteum, most notably in the phalanges and clavicles.
Distractors:
Lamellar bone in a mosaic pattern: Seen in Paget’s disease of bone.
Osteoid matrix accumulation: Seen in osteomalacia/rickets (vitamin D deficiency).
Spongiosa filling medullary canals: Seen in osteopetrosis.
Trabecular thinning: Characteristic of postmenopausal osteoporosis.
Hook: Classic radiographic finding is subperiosteal resorption of the radial aspect of the middle phalanges.
In advanced chronic kidney disease, secondary hyperparathyroidism is characterized by __________ PTH, __________ calcium, and __________ phosphorus.
Correct Answers: Increased; Low/Normal; High
Mini-Explanation: Impaired phosphate excretion causes hyperphosphatemia, which lowers calcium and directly stimulates PTH release. The kidneys also fail to produce active vitamin D, worsening hypocalcemia.
Distractors:
Increased PTH, High Calcium, Low Phosphorus: Primary hyperparathyroidism.
Decreased PTH, Low Calcium, High Phosphorus: Hypoparathyroidism.
Increased PTH, Low Calcium, Low Phosphorus: Vitamin D deficiency.
Decreased PTH, High Calcium, High Phosphorus: Granulomatous disease (e.g., sarcoidosis) or vitamin D intoxication.
Hook: Remember the sequence: CKD → ↑PO₄ → ↓Ca²⁺ → ↑PTH. This is the classic CKD-MBD (Mineral and Bone Disorder) lab pattern.
Erythropoietin, which is deficient in chronic kidney disease, is primarily produced by __________ in the kidney.
Correct Answer: Peritubular interstitial cells (fibroblasts)
Mini-Explanation: These specialized cells in the renal cortex sense hypoxia and produce erythropoietin to stimulate red blood cell production in the bone marrow.
Distractors:
Efferent arteriolar smooth muscles: Part of the juxtaglomerular apparatus; involved in renin secretion, not EPO.
Glomerular podocytes: Form the filtration barrier; not involved in hormone production.
Juxtaglomerular cells: Secrete renin.
Proximal tubule epithelium: Reabsorbs solutes; does not produce EPO.
Hook: The anemia of CKD is normocytic and is primarily due to the loss of these EPO-producing cells as the kidney parenchyma is destroyed.
Humoral hypercalcemia of malignancy, often from squamous cell lung cancer, is most commonly caused by tumor secretion of __________.
Correct Answer: PTH-related protein (PTHrP)
Mini-Explanation: PTHrP mimics PTH, causing increased bone resorption and renal calcium reabsorption, but it does not stimulate 1,25-(OH)₂ vitamin D production.
Distractors:
1,25-dihydroxyvitamin D: Elevated in granulomatous diseases (e.g., sarcoidosis) and some lymphomas.
ACTH: Causes Cushing’s syndrome, not hypercalcemia.
Parathyroid hormone (PTH): Suppressed in malignancy; elevated in primary hyperparathyroidism.
Phosphorus: Typically low or normal in humoral hypercalcemia due to PTHrP’s phosphaturic effect.
Hook: PTHrP-mediated hypercalcemia presents with a suppressed PTH level, distinguishing it from primary hyperparathyroidism.
An age-related decline in renal function is primarily due to a decreased number of functional __________.
Correct Answer: Glomeruli
Mini-Explanation: Normal aging involves glomerulosclerosis, reducing the number of functional filtering units and leading to a lower GFR and creatinine clearance.
Distractors:
Decreased solute excreting ability: Solute excretion is relatively preserved; concentrating ability is impaired.
Increased creatinine clearance: Creatinine clearance decreases with age.
Increased renal blood flow: Renal blood flow decreases with age.
Increased sensitivity for renin release: Renin secretion and responsiveness decrease with age.
Hook: This is why a “normal” serum creatinine in an elderly person can be misleading, as it may reflect significantly reduced muscle mass masking a truly low GFR.
An infusion of angiotensin II increases glomerular net filtration pressure by causing __________ arteriolar constriction.
Correct Answer: Efferent
Mini-Explanation: Selective efferent constriction increases the hydrostatic pressure upstream in the glomerular capillaries, which is the main driving force for filtration.
Distractors:
Albumin: Increases capillary oncotic pressure, decreasing net filtration pressure.
Alpha-1 agonist: Causes afferent constriction, decreasing hydrostatic pressure and GFR.
Prostaglandin synthesis inhibitor: Causes afferent constriction, decreasing hydrostatic pressure and GFR.
Vasopressin V2 antagonist: Causes water loss (diuresis), which can reduce blood volume and hydrostatic pressure.
Hook: This is the key mechanism for maintaining GFR during reduced renal perfusion (e.g., in heart failure).
Primary nocturnal enuresis (bed-wetting) in a child with a normal physical exam and urinalysis is most often due to a __________.
Correct Answer: Brain maturational delay
Mini-Explanation: It is a developmental delay in the central nervous system’s ability to sense a full bladder or inhibit the micturition reflex during sleep.
Distractors:
Bladder flaccidity: Causes overflow incontinence and a weak stream, not seen here.
Increased bladder capacity: Enuresis is more commonly associated with a smaller functional bladder capacity.
Osmotic diuresis: Would cause polyuria and likely glucosuria (e.g., diabetes mellitus).
Posterior urethral valves: Causes obstructive symptoms (weak stream) and daytime incontinence.
Hook: This is a diagnosis of exclusion. A strong, continuous stream and normal daytime voiding help rule out anatomical or neurological causes.
Post-thyroidectomy hypocalcemia, due to hypoparathyroidism, is caused by decreased bone resorption and decreased renal __________.
Correct Answer: Calcium reabsorption
Mini-Explanation: Lack of PTH removes its stimulatory effect on calcium reabsorption in the distal convoluted tubule, leading to urinary calcium loss and worsening hypocalcemia.
Distractors:
Calcium binding by albumin: Causes pseudohypocalcemia; total calcium is low but ionized (active) calcium is normal.
Calcium release from bones: This is decreased due to lack of PTH.
Hydroxylation of vitamin D: This is decreased due to lack of PTH, reducing intestinal calcium absorption.
Intestinal phosphate absorption: This is decreased due to low active vitamin D.
Hook: The mnemonic for PTH actions on the kidney is: “PTH turns on the Canal and turns off the Phosphate canal.”
Post-thyroidectomy hypocalcemia, due to hypoparathyroidism, is caused by decreased bone resorption and decreased renal __________.
Correct Answer: Calcium reabsorption
Mini-Explanation: Lack of PTH removes its stimulatory effect on calcium reabsorption in the distal convoluted tubule, leading to urinary calcium loss and worsening hypocalcemia.
Distractors:
Calcium binding by albumin: Causes pseudohypocalcemia; total calcium is low but ionized (active) calcium is normal.
Calcium release from bones: This is decreased due to lack of PTH.
Hydroxylation of vitamin D: This is decreased due to lack of PTH, reducing intestinal calcium absorption.
Intestinal phosphate absorption: This is decreased due to low active vitamin D.
Hook: The mnemonic for PTH actions on the kidney is: “PTH turns on the Canal and turns off the Phosphate canal.”
Constriction of the __________ arteriole decreases renal plasma flow but increases glomerular hydrostatic pressure and filtration fraction.
Correct Answer: Efferent
Mini-Explanation: Efferent constriction increases resistance, reducing overall flow (RPF) but raising pressure upstream in the glomerulus, which drives more filtration (increasing FF).
Distractors:
Hyperproteinemia: Increases capillary oncotic pressure, decreasing GFR and FF.
Bladder neck obstruction: Increases Bowman’s space pressure, decreasing GFR and FF.
Constriction of the afferent arteriole: Decreases both RPF and glomerular pressure, decreasing GFR; FF remains relatively unchanged.
Dilation of the efferent arteriole: Increases RPF but decreases glomerular pressure, decreasing GFR and FF.
Hook: This is the mechanism of action of angiotensin II, which helps maintain GFR during states of low renal perfusion.
Chronic renal hypoperfusion (e.g., from renal artery stenosis) causes hyperplasia of the juxtaglomerular cells, which are modified __________ cells.
Correct Answer: Smooth muscle
Mini-Explanation: These specialized cells in the wall of the afferent arteriole produce and secrete renin in response to low perfusion pressure.
Distractors:
Cuboidal epithelial cells of the proximal tubules: Damage from ischemia leads to atrophy/necrosis, not hyperplasia.
Endothelial cells of the afferent arterioles: The endothelium lines the vessel but is not the primary renin-producing cell.
Intraglomerular mesangial cells: These are involved in glomerular filtration and immune complex clearance, not renin production.
Squamous epithelial cells of the thick ascending limb: These are part of the macula densa, which senses tubular flow, but does not undergo hyperplasia.
Hook: This hyperplasia is part of the pathological feedback loop in renovascular hypertension, leading to excessive renin production.
Compared to the true glomerular filtration rate (GFR), the calculated creatinine clearance is an overestimate because creatinine undergoes tubular __________.
Correct Answer: Secretion
Mini-Explanation: A small amount of creatinine is actively secreted by the proximal tubule, adding to the amount that was filtered, causing the clearance value to overestimate the true GFR.
Distractors:
20% higher: This is the magnitude of the overestimation, not the mechanism.
20% lower / 80% lower: Creatinine clearance overestimates GFR, it is not an underestimate.
90% higher: This degree of overestimation is too large; the typical overestimation is 10-20%.
Equal: This would only be true if there were no tubular handling of creatinine.
Hook: This is why GFR estimating equations (e.g., CKD-EPI) are more accurate than a raw creatinine clearance calculation.
