Four days after admission to the hospital for anorexia nervosa, a 20-year-old woman has new-onset palpitations and paresthesias in all four limbs. Prior to admission, she was found unconscious by her parents on the floor of a residential treatment center. The patient was started on a trial of nutritional rehabilitation upon arrival to the hospital. Her temperature is 36°C (96.8°F), pulse is 47/min, and blood pressure is 90/60 mmHg. She is 160 cm tall and weighs 35 kg; BMI is 14 kg/m2. The patient appears emaciated. Examination shows lower leg edema. A 2/6 holosystolic murmur is heard over the 5th intercostal space at the midclavicular line. An ECG shows intermittent supraventricular tachycardia and QTc prolongation. Serum studies show: Day 2 Day 4 Potassium (mEq/L) 3.5 2.7 Calcium (mg/dL) 8.5 7.8 Magnesium (mEq/L) 1.2 0.5 Phosphorus (mg/dL) 3.6 1.5 Which of the following is the most likely underlying cause of this patient's condition?

Uncompensated metabolic alkalosis

On cardiology service rounds, your team sees a patient admitted with an acute congestive heart failure exacerbation. In congestive heart failure, decreased cardiac function leads to decreased renal perfusion, which eventually leads to excess volume retention. To test your knowledge of physiology, your attending asks you which segment of the nephron is responsible for the majority of water absorption. Which of the following is a correct pairing of the segment of the nephron that reabsorbs the majority of all filtered water with the means by which that segment absorbs water?

Proximal convoluted tubule via passive diffusion following ion reabsorption

Distal convoluted tubule via passive diffusion following ion reabsorption

Distal convoluted tubule via aquaporin channels

Thick ascending loop of Henle via passive diffusion following ion reabsorption

Collecting duct via aquaporin channels

A new drug has been shown to block epithelial sodium channels in the cortical collecting duct. Which of the following is most likely to be decreased upon drug administration?

Potassium secretion in the collecting tubules

Urea reabsorption in the collecting tubules

Hydrogen ion secretion in the collecting tubules

Sodium secretion in the collecting tubules

Sodium chloride reabsorption in the distal tubule

A 55-year-old woman presents to the emergency room with severe abdominal pain for the past 24 hours. She has also noticed blood in her urine. She does not have any significant past medical history. Family history is significant for her mother having cholecystitis status post cholecystectomy at age 45. Her vital signs include: temperature 36.8°C (98.2°F), pulse 103/min, respiratory rate 15/min, blood pressure 105/85 mm Hg. Physical examination is significant for a woman continuously moving on the exam table in an attempt to get comfortable. Laboratory findings are significant for the following: Serum electrolytes Na 138 mEq/L N: 135–145 mEq/L K 4.0 mEq/L N: 3.5–5.0 mEq/L Cl 102 mEq/L N: 98–108 mEq/L CO2 27 mEq/L N: 22–32 mEq/L Ca 9.2 mEq/dL N: 8.4–10.2 mEq/dL PO4 3.5 mg/dL N: 3.0–4.5 mg/dL A 24-hour urine collection is performed and reveals a urinary calcium of 345 mg/day (ref: < 300 mg/day in men; < 250 mg/day in women). A non-contrast CT of the abdomen is performed and is shown in the exhibit. The patient’s symptoms pass within the next 12 hours with hydration and acetaminophen for pain management. She is prescribed a medication to prevent subsequent episodes. At which of the following parts of the nephron does this medication most likely work?

Descending limb of the loop of Henle

Thick ascending limb of the loop of Henle

A 55-year-old man presents to his physician with weakness and fatigue for 1 week. There is no significant past medical history. He mentions that he is very health conscious and has heard about the health benefits of juices. He is following a juice-only diet for the last 2 weeks. His physical examination is completely normal, except for depressed deep tendon reflexes. The only abnormality in a complete laboratory evaluation is a serum potassium level of 6.0 mEq/L (6.0 mmol/L). There are significantly peaked T-waves on ECG. Which of the following pathophysiologic mechanisms best explains the patient’s symptoms?

Decreased resting membrane potential of skeletal muscle cells

Prolonged release of Ca2+ ions after stimulation of Ryanodine receptors

Hyperpolarization of skeletal muscle cells

Dysfunction of dystrophin-glycoprotein complex

A 28-year-old woman presents to her primary care physician with recurring muscle cramps that have lasted for the last 2 weeks. She mentions that she commonly has these in her legs and back. She also has a constant tingling sensation around her mouth. On physical examination, her vital signs are stable. The Trousseau sign and Chvostek sign are present with exaggerated deep tendon reflexes. A comprehensive blood test reveals the following: Na+ 140 mEq/L K+ 4.5 mEq/L Chloride 100 mEq/L Bicarbonate 24 mEq/L Creatinine 0.9 mg/dL Ca2+ 7.0 mg/dL Which of the following electrophysiologic mechanisms best explain this woman’s clinical features?

Decreased firing threshold for action potential

Reduction of afterhyperpolarization

Inhibition of sodium current through sodium leak channels (NALCN)

Inhibition of Na+ and Ca2+ currents through cyclic nucleotide-gated (CNG) channels

Stimulation of GABA (γ-aminobutyric acid) receptors

Magnesium handling for USMLE Step 3: High-Yield Physiology Lessons

Mg²⁺ Reabsorption Sites - The Magnesium Journey

Renal Magnesium Handling in PCT, TALH, and DCT

Magnesium reabsorption is primarily a passive, paracellular process driven by electrochemical gradients established by other transporters. The Thick Ascending Limb is the key site.

Thick Ascending Limb (TAL): The major site of Mg²⁺ reabsorption (60-70%).
- Mechanism: Passive, paracellular movement driven by the lumen-positive potential created by the Na-K-2Cl (NKCC2) cotransporter.
- 📌 Thick Ascending Limb Loves Mg²⁺ (TALL-Mg).
Distal Convoluted Tubule (DCT): Reabsorbs 5-10% via active transcellular transport through TRPM6 channels.

⭐ Loop diuretics inhibit the NKCC2 cotransporter in the TAL, which ↓ the lumen-positive potential. This impairs paracellular reabsorption of both Mg²⁺ and Ca²⁺, leading to hypomagnesemia and hypocalcemia.

Regulation of Reabsorption - Magnesium's Master Controls

Primary Regulatory Site: Thick Ascending Limb (TAL) of Henle's loop, reabsorbing 60-70% of filtered $Mg^{2+}$ via a paracellular route.
- This transport depends on a lumen-positive potential generated by the Na-K-2Cl cotransporter (NKCC2) and ROMK channel.
- Proteins claudin-16 and claudin-19 form the paracellular channels.
Factors ↑ Reabsorption:
- Parathyroid Hormone (PTH): Upregulates TAL $Mg^{2+}$ transport.
- Hypomagnesemia & Hypocalcemia: Decrease activation of the CaSR, promoting reabsorption.
Factors ↓ Reabsorption (leading to Mg²⁺ wasting):
- Hypermagnesemia & Hypercalcemia: Activate the basolateral Calcium-Sensing Receptor (CaSR), inhibiting reabsorption.
- Loop Diuretics (e.g., Furosemide): Inhibit NKCC2, reducing the driving force for paracellular transport.
- Volume Expansion.
- Metabolic Acidosis.

⭐ Exam Favorite: Activating mutations in the Calcium-Sensing Receptor (CaSR) cause familial hypocalcemia with hypercalciuria, but also lead to significant hypomagnesemia due to suppressed TAL reabsorption.

Clinical Correlations - Too Much, Too Little

Hypermagnesemia (>2.2 mEq/L)
- Causes: Chronic kidney disease, iatrogenic (e.g., MgSO₄ for eclampsia, antacids/laxatives).
- Symptoms: ↓ Deep tendon reflexes (DTRs), lethargy, respiratory depression, bradycardia, cardiac arrest.
- Treatment: IV Calcium Gluconate (antagonizes Mg²⁺ effects), saline diuresis, possible hemodialysis.
Hypomagnesemia (<1.8 mEq/L)
- Causes: Diuretics (loop & thiazide), PPIs, alcoholism, diarrhea, amphotericin B.
- Symptoms: Neuromuscular hyperexcitability (tetany, fasciculations, seizures), arrhythmias (Torsades de Pointes).
⭐ Hypomagnesemia frequently causes refractory hypokalemia and hypocalcemia. It impairs PTH release and function, and promotes renal K⁺ wasting.

ECG: Torsades de Pointes and MgSO4 treatment

High‑Yield Points - ⚡ Biggest Takeaways

~60-70% of filtered Mg²⁺ is reabsorbed in the thick ascending limb (TAL), mainly via the paracellular route.

Transport is driven by the lumen-positive potential generated by the Na-K-2Cl (NKCC2) cotransporter.

Loop diuretics (e.g., Furosemide) are a major cause of hypomagnesemia by inhibiting the NKCC2.

The Calcium-Sensing Receptor (CaSR) activation by high Ca²⁺ or Mg²⁺ inhibits reabsorption in the TAL.

Hypomagnesemia can cause refractory hypokalemia and hypocalcemia.

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