A 65-year-old man is brought to the emergency department by ambulance after falling during a hiking trip. He was hiking with friends when he fell off a 3 story ledge and was not able to be rescued until 6 hours after the accident. On arrival, he is found to have multiple lacerations as well as a pelvic fracture. His past medical history is significant for diabetes and benign prostatic hyperplasia, for which he takes metformin and prazosin respectively. Furthermore, he has a family history of autoimmune diseases. Selected lab results are shown below:
Serum:
Na+: 135 mEq/L
Creatinine: 1.5 mg/dL
Blood urea nitrogen: 37 mg/dL
Urine:
Na+: 13.5 mEq/L
Creatinine: 18 mg/dL
Osmolality: 580 mOsm/kg
Which of the following is the most likely cause of this patient's increased creatinine level?
Q42
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?
Q43
A scientist is studying the excretion of a novel toxin X by the kidney in order to understand the dynamics of this new substance. He discovers that this new toxin X has a clearance that is half that of inulin in a particular patient. This patient's filtration fraction is 20% and his para-aminohippuric acid (PAH) dynamics are as follows:
Urine volume: 100 mL/min
Urine PAH concentration: 30 mg/mL
Plasma PAH concentration: 5 mg/mL
Given these findings, what is the clearance of the novel toxin X?
Q44
A 41-year-old man presents at an office for a regular health check-up. He has no complaints. He has no history of significant illnesses. He currently takes omeprazole for gastroesophageal reflux disease. He occasionally smokes cigarettes and drinks alcohol. The family history is unremarkable. The vital signs include: blood pressure 133/67 mm Hg, pulse 67/min, respiratory rate 15/min, and temperature 36.7°C (98.0°F). The physical examination was within normal limits. A complete blood count reveals normal values. A urinalysis was ordered which shows the following:
pH 6.7
Color light yellow
RBC none
WBC none
Protein absent
Cast hyaline casts
Glucose absent
Crystal none
Ketone absent
Nitrite absent
Which of the following is the likely etiology for hyaline casts in this patient?
Q45
A 17-year-old boy comes to the physician for a follow-up examination. Two months ago, he suffered a spinal fracture after a fall from the roof. He feels well. His father has multiple endocrine neoplasia type 1. Vital signs are within normal limits. Examination shows no abnormalities. Laboratory studies show:
Hemoglobin 13.7 g/dL
Serum
Creatinine 0.7 mg/dL
Proteins
Total 7.0 g/dL
Albumin 4.1 g/dL
Calcium 11.4 mg/dL
Phosphorus 5.3 mg/dL
Alkaline phosphatase 100 U/L
Which of the following is the most likely cause of these findings?
Q46
A 58-year old man comes to his physician because of a 1-month history of increased thirst and nocturia. He is drinking a lot of water to compensate for any dehydration. His brother has type 2 diabetes mellitus. Physical examination shows dry mucous membranes. Laboratory studies show a serum sodium of 151 mEq/L and glucose of 121 mg/dL. A water deprivation test shows:
Serum osmolality
(mOsmol/kg H2O) Urine osmolality
(mOsmol/kg H2O)
Initial presentation 295 285
After 3 hours without fluids 305 310
After administration of antidiuretic hormone (ADH) analog 280 355
Which of the following is the most likely diagnosis?
Q47
A 24-year-old man is hospitalized for an elective gastrointestinal surgery 24 hours before the scheduled day of surgery. The surgeon has ordered food and fluids to be withheld from the patient from 12 hours before the surgery and the administration of intravenous isotonic saline. Based on his body weight, his fluid requirement for 12 hours is 900 mL. However, the following day, the surgeon finds that 3 pints of isotonic fluid (1 pint = 500 mL) were administered over the preceding last 12 hours. Which of the following options best describes the resulting changes in the volume of intracellular fluid (ICF) and the body osmolality of the patient?
Q48
A 52-year-old man with chronic kidney disease presents for significant back pain that has gotten worse in the past 2 days. On exam, the patient has a moderate kyphosis with decreased range of motion of the spine secondary to pain. The patient has no neurologic deficits but is in severe pain. Lab work reveals a low normal serum calcium, slightly increased serum phosphate, and decreased serum vitamin D. What is the cause of this patient’s presentation?
Q49
A 33-year-old man comes to the physician with his wife for evaluation of infertility. They have been unable to conceive for 2 years. The man reports normal libido and erectile function. He has smoked one pack of cigarettes daily for 13 years. He does not take any medications. He has a history of right-sided cryptorchidism that was surgically corrected when he was 7 years of age. Physical examination shows no abnormalities. Analysis of his semen shows a low sperm count. Laboratory studies are most likely to show which of the following?
Q50
A 34-year-old man presents to his primary care physician with frequent urination. He was recently hospitalized following a severe motorcycle accident in which he suffered multiple injuries to his head and extremities. He reports that he has been constantly thirsty and has been urinating four to five times per night since being discharged from the hospital one week prior to presentation. His past medical history is notable for type II diabetes mellitus, which is well controlled on metformin. He has a 10 pack-year smoking history and drinks 3-4 alcoholic beverages per day. His temperature is 98.8°F (37.1°C), blood pressure is 110/70 mmHg, pulse is 95/min, and respirations are 18/min. Physical examination reveals delayed capillary refill and decreased skin turgor. Notable laboratory results are shown below:
Serum:
Na+: 148 mEq/L
Cl-: 101 mEq/L
K+: 3.7 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 110 mg/dL
Hemoglobin A1c: 5.7%
This patient’s condition is most likely caused by defective production in which of the following locations?
Renal US Medical PG Practice Questions and MCQs
Question 41: A 65-year-old man is brought to the emergency department by ambulance after falling during a hiking trip. He was hiking with friends when he fell off a 3 story ledge and was not able to be rescued until 6 hours after the accident. On arrival, he is found to have multiple lacerations as well as a pelvic fracture. His past medical history is significant for diabetes and benign prostatic hyperplasia, for which he takes metformin and prazosin respectively. Furthermore, he has a family history of autoimmune diseases. Selected lab results are shown below:
Serum:
Na+: 135 mEq/L
Creatinine: 1.5 mg/dL
Blood urea nitrogen: 37 mg/dL
Urine:
Na+: 13.5 mEq/L
Creatinine: 18 mg/dL
Osmolality: 580 mOsm/kg
Which of the following is the most likely cause of this patient's increased creatinine level?
A. Autoimmune disease
B. Compression of urethra by prostate
C. Blood loss (Correct Answer)
D. Diabetic nephropathy
E. Rhabdomyolysis
Explanation: ***Correct: Blood loss***
- The fall from a 3-story ledge and subsequent **pelvic fracture** indicate a high likelihood of significant **internal bleeding** and **hypovolemia**, leading to decreased renal perfusion and a pre-renal acute kidney injury (AKI) as evidenced by the elevated BUN/creatinine ratio (37/1.5 = 24.7), low urine sodium, and high urine osmolality.
- **Hypovolemia** from blood loss is a common cause of **pre-renal AKI**, characterized by the kidneys attempting to conserve fluid, resulting in concentrated urine with low sodium.
*Incorrect: Autoimmune disease*
- While a family history of autoimmune diseases exists, there is no direct evidence in the current presentation (e.g., specific markers, symptoms) to suggest an **autoimmune nephritis** as the acute cause of his renal dysfunction.
- Autoimmune causes of kidney injury typically present with proteinuria, hematuria, or other systemic inflammatory signs, which are not described.
*Incorrect: Compression of urethra by prostate*
- Although the patient has benign prostatic hyperplasia (BPH) and takes prazosin (an alpha-blocker to treat BPH), their current presentation of **pre-renal AKI** with concentrated urine does not fit typical **post-renal obstruction**.
- **Post-renal obstruction** from BPH would typically present with symptoms like difficulty urinating, elevated bladder volume, and often hydronephrosis, none of which are indicated here as the primary cause of acute renal failure.
*Incorrect: Diabetic nephropathy*
- The patient's history of diabetes makes **diabetic nephropathy** a potential cause of chronic kidney disease, but the **acute increase in creatinine** following a traumatic event strongly suggests an acute insult rather than a sudden exacerbation of chronic diabetic kidney disease.
- **Diabetic nephropathy** usually develops over years, presenting with proteinuria and a gradual decline in GFR, not an acute surge in creatinine following an injury, and it does not explain the pre-renal parameters seen in the urine.
*Incorrect: Rhabdomyolysis*
- While a severe fall could potentially cause **rhabdomyolysis** (muscle breakdown), the provided lab values do not include elevated **creatine kinase**, which is the hallmark of this condition.
- Although rhabdomyolysis can cause AKI, the **pre-renal parameters** (high BUN/Cr ratio, low urine Na, high urine osmolality) are more consistent with hypovolemia from blood loss rather than direct tubular injury from myoglobin.
Question 42: 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?
A. Distal convoluted tubule via passive diffusion following ion reabsorption
B. Distal convoluted tubule via aquaporin channels
C. Thick ascending loop of Henle via passive diffusion following ion reabsorption
D. Proximal convoluted tubule via passive diffusion following ion reabsorption (Correct Answer)
E. Collecting duct via aquaporin channels
Explanation: ***Proximal convoluted tubule via passive diffusion following ion reabsorption***
- The **proximal convoluted tubule (PCT)** is responsible for reabsorbing approximately **65-70% of filtered water**, making it the primary site of water reabsorption in the nephron.
- This water reabsorption primarily occurs **passively**, following the active reabsorption of solutes (especially **sodium ions**), which creates an osmotic gradient.
*Distal convoluted tubule via passive diffusion following ion reabsorption*
- The **distal convoluted tubule (DCT)** reabsorbs a much smaller percentage of filtered water (around 5-10%) and its water reabsorption is largely **regulated by ADH**, not primarily simple passive diffusion following bulk ion reabsorption.
- While some passive water movement occurs, it is not the main mechanism or location for the majority of water reabsorption.
*Distal convoluted tubule via aquaporin channels*
- While aquaporin channels do play a role in water reabsorption in the DCT, particularly under the influence of **ADH**, the DCT is not the segment responsible for the **majority of all filtered water absorption**.
- The bulk of water reabsorption occurs earlier in the nephron, independently of ADH for the most part.
*Thick ascending loop of Henle via passive diffusion following ion reabsorption*
- The **thick ascending loop of Henle** is primarily involved in reabsorbing ions like Na+, K+, and Cl- but is largely **impermeable to water**.
- Its impermeability to water is crucial for creating the **osmotic gradient** in the renal medulla, which is necessary for later water reabsorption.
*Collecting duct via aquaporin channels*
- The **collecting duct** is critically important for **regulated water reabsorption** via **aquaporin-2 channels** under the influence of **ADH**, allowing for fine-tuning of urine concentration.
- However, it reabsorbs only a variable portion (typically 5-19%) of the remaining filtered water, not the **majority of all filtered water**.
Question 43: A scientist is studying the excretion of a novel toxin X by the kidney in order to understand the dynamics of this new substance. He discovers that this new toxin X has a clearance that is half that of inulin in a particular patient. This patient's filtration fraction is 20% and his para-aminohippuric acid (PAH) dynamics are as follows:
Urine volume: 100 mL/min
Urine PAH concentration: 30 mg/mL
Plasma PAH concentration: 5 mg/mL
Given these findings, what is the clearance of the novel toxin X?
A. 1,500 mL/min
B. 600 mL/min
C. 300 mL/min
D. 60 mL/min (Correct Answer)
E. 120 mL/min
Explanation: ***60 ml/min***
- First, calculate the **renal plasma flow (RPF)** using PAH clearance: RPF = (Urine PAH conc. × Urine vol.) / Plasma PAH conc. = (30 mg/mL × 100 mL/min) / 5 mg/mL = 600 mL/min.
- Next, calculate the **glomerular filtration rate (GFR)**, which is the clearance of inulin. GFR = RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min. Toxin X clearance is half of inulin clearance, so 120 mL/min / 2 = **60 mL/min**.
*1,500 ml/min*
- This value is likely obtained if an incorrect formula or conversion was made, possibly by misinterpreting the units or the relationship between GFR, RPF, and filtration fraction.
- It significantly overestimates the clearance for a substance that is cleared at half the rate of inulin.
*600 ml/min*
- This value represents the **renal plasma flow (RPF)**, calculated using the PAH clearance data.
- It does not account for the filtration fraction or the fact that toxin X clearance is half of inulin clearance (GFR).
*300 ml/min*
- This value would be obtained if the renal plasma flow (RPF) was incorrectly halved, or if an intermediate calculation was misinterpreted as the final answer.
- It does not align with the given filtration fraction and the relationship between toxin X and inulin clearance.
*120 ml/min*
- This value represents the **glomerular filtration rate (GFR)**, which is equal to the clearance of inulin (RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min).
- The question states that the clearance of toxin X is **half** that of inulin, so this is an intermediate step, not the final answer.
Question 44: A 41-year-old man presents at an office for a regular health check-up. He has no complaints. He has no history of significant illnesses. He currently takes omeprazole for gastroesophageal reflux disease. He occasionally smokes cigarettes and drinks alcohol. The family history is unremarkable. The vital signs include: blood pressure 133/67 mm Hg, pulse 67/min, respiratory rate 15/min, and temperature 36.7°C (98.0°F). The physical examination was within normal limits. A complete blood count reveals normal values. A urinalysis was ordered which shows the following:
pH 6.7
Color light yellow
RBC none
WBC none
Protein absent
Cast hyaline casts
Glucose absent
Crystal none
Ketone absent
Nitrite absent
Which of the following is the likely etiology for hyaline casts in this patient?
A. End-stage renal disease/chronic kidney disease (CKD)
B. Acute interstitial nephritis
C. Non-specific; can be a normal finding (Correct Answer)
D. Post-streptococcal glomerulonephritis
E. Nephrotic syndrome
Explanation: ***Non-specific; can be a normal finding***
- **Hyaline casts** are formed from precipitated **Tamm-Horsfall mucoprotein** and can be seen in small numbers even in **healthy individuals**, particularly with concentrated urine, dehydration, or strenuous exercise.
- In this patient, the absence of other red flags (normal vitals, normal CBC, no symptoms, and other clear urinalysis findings) makes the presence of hyaline casts a **non-specific finding** and likely benign.
*End-stage renal disease/chronic kidney disease (CKD)*
- While CKD can feature various casts, **broad waxy casts** are more characteristic of advanced and chronic kidney damage, indicating significant tubular dilation.
- The patient's **normal vital signs** and **absence of symptoms or other lab abnormalities** make advanced renal disease unlikely.
*Acute interstitial nephritis*
- **Acute interstitial nephritis** is typically associated with a history of **drug exposure** (e.g., penicillin, NSAIDs, PPIs) and presents with **eosinophiluria**, **white blood cell casts**, and systemic symptoms like fever or rash, none of which are present here.
- The patient is on omeprazole, which can rarely cause AIN, but the **lack of symptoms** (e.g., fever, rash, eosinophilia) and typical findings (e.g. WBC casts) makes it less likely.
*Post-streptococcal glomerulonephritis*
- This condition typically follows a **streptococcal infection** and presents with signs of acute nephritis, including **hematuria**, **red blood cell casts**, and **proteinuria**, which are absent in this urinalysis.
- The patient has **no history of recent infection** or associated symptoms like edema or hypertension.
*Nephrotic syndrome*
- **Nephrotic syndrome** is characterized by **massive proteinuria** (>3.5 g/day), **hypoalbuminemia**, edema, and hyperlipidemia.
- The urinalysis shows **absent protein**, ruling out nephrotic syndrome.
Question 45: A 17-year-old boy comes to the physician for a follow-up examination. Two months ago, he suffered a spinal fracture after a fall from the roof. He feels well. His father has multiple endocrine neoplasia type 1. Vital signs are within normal limits. Examination shows no abnormalities. Laboratory studies show:
Hemoglobin 13.7 g/dL
Serum
Creatinine 0.7 mg/dL
Proteins
Total 7.0 g/dL
Albumin 4.1 g/dL
Calcium 11.4 mg/dL
Phosphorus 5.3 mg/dL
Alkaline phosphatase 100 U/L
Which of the following is the most likely cause of these findings?
A. Immobilization (Correct Answer)
B. Parathyroid adenoma
C. Paraneoplastic syndrome
D. Sarcoidosis
E. Pseudohypercalcemia
Explanation: ***Immobilization***
- Prolonged **immobilization**, especially after a spinal fracture, leads to **bone resorption**, releasing calcium and phosphorus into the bloodstream, causing **hypercalcemia** and **hyperphosphatemia**.
- Though calcium and phosphorus are elevated, the **alkaline phosphatase** is normal, which is consistent with immobilization-induced bone resorption rather than primary bone disease.
*Parathyroid adenoma*
- A **parathyroid adenoma** causes primary **hyperparathyroidism**, characterized by **hypercalcemia** and **hypophosphatemia** (due to increased renal phosphate excretion), which contradicts the elevated phosphorus level seen here.
- Although the father has MEN1, a personal history of parathyroid adenoma is not indicated by the lab results.
*Paraneoplastic syndrome*
- **Paraneoplastic syndrome** causing hypercalcemia is typically due to ectopic production of **parathyroid hormone-related peptide (PTHrP)**, leading to **hypercalcemia** with **low PTH** and generally **low phosphorus** levels.
- This condition most commonly occurs with malignancies, such as squamous cell carcinoma, which is not indicated in this healthy-appearing young man with a recent fracture.
*Sarcoidosis*
- **Sarcoidosis** causes hypercalcemia due to increased synthesis of **1,25-dihydroxyvitamin D** by activated macrophages, leading to increased intestinal calcium absorption.
- This typically results in **hypercalcemia** with **normal or low PTH** and **normal or low phosphorus** levels; it is not associated with elevated phosphorus.
*Pseudohypercalcemia*
- **Pseudohypercalcemia** is an artifactual elevation of total calcium, usually due to **severe dehydration** or **elevated protein** levels, particularly **albumin** or **immunoglobulins**.
- In this case, the albumin and total protein levels are within the normal range, making pseudohypercalcemia unlikely.
Question 46: A 58-year old man comes to his physician because of a 1-month history of increased thirst and nocturia. He is drinking a lot of water to compensate for any dehydration. His brother has type 2 diabetes mellitus. Physical examination shows dry mucous membranes. Laboratory studies show a serum sodium of 151 mEq/L and glucose of 121 mg/dL. A water deprivation test shows:
Serum osmolality
(mOsmol/kg H2O) Urine osmolality
(mOsmol/kg H2O)
Initial presentation 295 285
After 3 hours without fluids 305 310
After administration of antidiuretic hormone (ADH) analog 280 355
Which of the following is the most likely diagnosis?
A. Nephrogenic diabetes insipidus
B. Partial central diabetes insipidus (Correct Answer)
C. Primary polydipsia
D. Osmotic diuresis
E. Complete central diabetes insipidus
Explanation: ***Partial central diabetes insipidus***
- The **initial water deprivation test** results (serum osmolality increasing to 305 mOsmol/kg H2O, urine osmolality remaining low at 310 mOsmol/kg H2O) indicate the kidneys are not concentrating urine maximally, suggesting **diabetes insipidus**.
- The subsequent **increase in urine osmolality from 310 to 355 mOsmol/kg H2O after ADH analog administration** (approximately **14% increase**) confirms that the kidneys can respond to ADH, indicating some endogenous ADH production but at insufficient levels, consistent with **partial central deficiency** of ADH.
- This modest response differentiates it from complete central DI (which would show >50% increase) and nephrogenic DI (which would show minimal or no response).
*Nephrogenic diabetes insipidus*
- In **nephrogenic diabetes insipidus**, the kidneys are **resistant to ADH**, meaning there would be little or no change in urine osmolality after the administration of an ADH analog (typically <10% increase).
- Here, the urine osmolality increased by approximately 14% after ADH administration, ruling out nephrogenic DI.
*Primary polydipsia*
- Patients with **primary polydipsia** would typically have **low serum sodium** due to excessive water intake and a low serum osmolality. This patient has **hypernatremia (151 mEq/L)**, which is inconsistent with primary polydipsia.
- The water deprivation test in primary polydipsia would show the ability to concentrate urine appropriately with increasing serum osmolality, as the ADH axis and kidney response are intact.
*Osmotic diuresis*
- **Osmotic diuresis** is characterized by elevated urine osmolality due to the excretion of osmotically active solutes (e.g., glucose, urea), leading to increased urine volume with normal ADH sensitivity.
- While the patient's glucose is slightly elevated (121 mg/dL), it is not high enough to cause significant osmotic diuresis, and the water deprivation test results specifically point to a problem with ADH regulation or response.
*Complete central diabetes insipidus*
- In **complete central diabetes insipidus**, the body produces **very little or no ADH**. This would manifest as a much lower initial urine osmolality after fluid deprivation (typically <300 mOsmol/kg H2O) and a much **more dramatic increase in urine osmolality** (often **>50% increase** or reaching >600 mOsmol/kg H2O) after ADH analog administration.
- The patient's modest increase from 310 to 355 mOsmol/kg H2O (14% increase) is consistent with a **partial** deficiency, not complete absence of ADH.
Question 47: A 24-year-old man is hospitalized for an elective gastrointestinal surgery 24 hours before the scheduled day of surgery. The surgeon has ordered food and fluids to be withheld from the patient from 12 hours before the surgery and the administration of intravenous isotonic saline. Based on his body weight, his fluid requirement for 12 hours is 900 mL. However, the following day, the surgeon finds that 3 pints of isotonic fluid (1 pint = 500 mL) were administered over the preceding last 12 hours. Which of the following options best describes the resulting changes in the volume of intracellular fluid (ICF) and the body osmolality of the patient?
A. No change in ICF volume, no change in body osmolality (Correct Answer)
B. Decreased ICF volume, no change in body osmolality
C. Increased ICF volume, decreased body osmolality
D. Increased ICF volume, no change in body osmolality
E. Decreased ICF volume, increased body osmolality
Explanation: ***No change in ICF volume, no change in body osmolality***
- An isotonic solution has the same osmolality as body fluids, meaning it **does not cause a net shift of water** between the intracellular and extracellular compartments.
- While an excess of 600 mL of isotonic fluid was administered (3 pints = 1500 mL; required = 900 mL), this excess primarily expands the **extracellular fluid volume (ECF)** without affecting cell volume or overall body osmolality in the short term.
*Decreased ICF volume, no change in body osmolality*
- ICF volume would only decrease if a **hypertonic solution** were administered, drawing water out of the cells.
- Since an isotonic solution was given, there is no osmotic gradient to cause water to shift out of the intracellular space.
*Increased ICF volume, decreased body osmolality*
- This scenario typically occurs with the administration of a **hypotonic solution**, which would lower the body's osmolality and cause water to shift into cells.
- The patient received an **isotonic solution**, which by definition does not alter body osmolality or cause water to move into cells.
*Increased ICF volume, no change in body osmolality*
- While the patient received an excess of fluid, the fluid was **isotonic**, meaning it does not create an osmotic gradient to drive water into the cells.
- An increase in ICF volume would imply a shift of fluid into the cells, which requires a decrease in ECF osmolality.
*Decreased ICF volume, increased body osmolality*
- This outcome would result from the administration of a **hypertonic solution** or severe dehydration, increasing ECF osmolality and drawing water out of the cells.
- The administration of an **isotonic solution** prevents such changes in osmolality and cell volume.
Question 48: A 52-year-old man with chronic kidney disease presents for significant back pain that has gotten worse in the past 2 days. On exam, the patient has a moderate kyphosis with decreased range of motion of the spine secondary to pain. The patient has no neurologic deficits but is in severe pain. Lab work reveals a low normal serum calcium, slightly increased serum phosphate, and decreased serum vitamin D. What is the cause of this patient’s presentation?
A. Drastic decrease in estrogen
B. Increased bone turnover
C. Increased calcium absorption in the intestines
D. Decreased production of calcitriol
E. Markedly increased PTH (Correct Answer)
Explanation: ***Markedly increased PTH***
- The patient's presentation of **chronic kidney disease** with low normal **calcium**, slightly increased **phosphate**, and decreased **vitamin D** suggests secondary hyperparathyroidism. This condition is characterized by an excessive production of **parathyroid hormone (PTH)** in response to chronic hypocalcemia, leading to renal osteodystrophy and symptoms such as severe back pain and skeletal deformities like kyphosis.
- Elevated PTH causes increased bone resorption, leading to bone pain and fragility, which aligns with the patient's severe back pain and kyphosis.
*Drastic decrease in estrogen*
- A drastic decrease in **estrogen** is typically associated with **postmenopausal osteoporosis** in women, which does not fit the profile of a 52-year-old male with chronic kidney disease.
- While men can experience age-related bone loss, a sudden and drastic decrease in estrogen is not the primary mechanism for bone disease in this clinical scenario.
*Increased bone turnover*
- **Increased bone turnover** is a general feature of many metabolic bone diseases, including osteoporosis and hyperparathyroidism. However, it is a consequence of the underlying pathology rather than the primary cause of this patient's specific presentation.
- While increased bone turnover is present in **secondary hyperparathyroidism**, this option does not explain the full spectrum of laboratory findings (low calcium, high phosphate, low vitamin D) and the pathophysiology specific to chronic kidney disease.
*Increased calcium absorption in the intestines*
- Patients with **chronic kidney disease** typically have **decreased calcium absorption** in the intestines due to impaired renal hydroxylation of **vitamin D** to its active form, **calcitriol**.
- Therefore, increased calcium absorption is inconsistent with the patient's underlying condition and laboratory findings in this case.
*Decreased production of calcitriol*
- **Decreased production of calcitriol** (1,25-dihydroxyvitamin D) is a key contributing factor to the development of secondary hyperparathyroidism in chronic kidney disease. This leads to reduced intestinal calcium absorption and contributes to hypocalcemia.
- While decreased calcitriol production is an important part of the pathophysiology, it is the resulting **markedly increased PTH** that directly drives the significant bone pain and skeletal abnormalities observed in the patient.
Question 49: A 33-year-old man comes to the physician with his wife for evaluation of infertility. They have been unable to conceive for 2 years. The man reports normal libido and erectile function. He has smoked one pack of cigarettes daily for 13 years. He does not take any medications. He has a history of right-sided cryptorchidism that was surgically corrected when he was 7 years of age. Physical examination shows no abnormalities. Analysis of his semen shows a low sperm count. Laboratory studies are most likely to show which of the following?
A. Decreased inhibin B concentration (Correct Answer)
B. Increased prolactin concentration
C. Increased placental ALP concentration
D. Decreased testosterone concentration
E. Decreased FSH concentration
Explanation: ***Decreased inhibin B concentration***
- The patient's history of **corrected cryptorchidism** and current **low sperm count** suggest primary testicular failure affecting spermatogenesis.
- **Inhibin B** is produced by Sertoli cells in response to FSH and is a marker of Sertoli cell function and spermatogenesis; its decrease indicates impaired spermatogenesis.
*Increased prolactin concentration*
- **Hyperprolactinemia** can cause hypogonadism and infertility by suppressing GnRH, leading to decreased LH, FSH, and testosterone.
- However, the patient has normal libido and erectile function, which would likely be affected by hyperprolactinemia.
*Increased placental ALP concentration*
- **Placental alkaline phosphatase (ALP)** is a marker for germ cell tumors, particularly seminomas.
- While cryptorchidism increases the risk of testicular cancer, the primary issue described is infertility due to low sperm count, not a presentation suggestive of a mass.
*Decreased testosterone concentration*
- Although **primary testicular failure** (Sertoli cell dysfunction) can lead to secondary Leydig cell dysfunction over time, the patient's normal libido and erectile function make an isolated or significant decrease in testosterone less likely as the primary finding.
- Leydig cells, which produce testosterone, are often less affected than Sertoli cells in cases of isolated spermatogenic failure following cryptorchidism.
*Decreased FSH concentration*
- **Decreased FSH** would suggest a central (hypothalamic or pituitary) cause of hypogonadism, leading to secondary testicular failure.
- In primary testicular failure (as suggested by cryptorchidism history), the pituitary compensates by **increasing FSH levels** due to the loss of negative feedback from inhibin B; therefore, FSH would be elevated, not decreased, in this patient.
Question 50: A 34-year-old man presents to his primary care physician with frequent urination. He was recently hospitalized following a severe motorcycle accident in which he suffered multiple injuries to his head and extremities. He reports that he has been constantly thirsty and has been urinating four to five times per night since being discharged from the hospital one week prior to presentation. His past medical history is notable for type II diabetes mellitus, which is well controlled on metformin. He has a 10 pack-year smoking history and drinks 3-4 alcoholic beverages per day. His temperature is 98.8°F (37.1°C), blood pressure is 110/70 mmHg, pulse is 95/min, and respirations are 18/min. Physical examination reveals delayed capillary refill and decreased skin turgor. Notable laboratory results are shown below:
Serum:
Na+: 148 mEq/L
Cl-: 101 mEq/L
K+: 3.7 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 110 mg/dL
Hemoglobin A1c: 5.7%
This patient’s condition is most likely caused by defective production in which of the following locations?
A. Anterior pituitary
B. Lateral nucleus of the hypothalamus
C. Supraoptic nucleus of the hypothalamus (Correct Answer)
D. Posterior pituitary
E. Posterior nucleus of the hypothalamus
Explanation: ***Supraoptic nucleus of the hypothalamus***
- This patient presents with symptoms of **central diabetes insipidus**, including **polyuria**, **polydipsia**, and signs of dehydration (high serum sodium, decreased skin turgor). Central diabetes insipidus is caused by a deficiency in the production or release of **antidiuretic hormone (ADH)**, also known as vasopressin.
- The **supraoptic nucleus** and the paraventricular nucleus of the hypothalamus are the primary sites of ADH synthesis. Damage to these nuclei, as could occur from a severe head injury in a motorcycle accident, would impair ADH production.
*Anterior pituitary*
- The **anterior pituitary** produces and secretes hormones such as growth hormone (GH), prolactin, ACTH, TSH, FSH, and LH, but it does **not produce ADH**.
- Dysfunction of the anterior pituitary would lead to different endocrine disorders, not central diabetes insipidus.
*Lateral nucleus of the hypothalamus*
- The **lateral nucleus of the hypothalamus** is primarily associated with **hunger and feeding behavior**. Damage to this area can lead to aphagia and weight loss.
- It plays no direct role in the production or secretion of **ADH**.
*Posterior pituitary*
- The **posterior pituitary** stores and releases **ADH and oxytocin**, but these hormones are synthesized in the hypothalamus.
- While damage to the posterior pituitary could impair ADH release, the question asks about defective *production*, which occurs in the hypothalamus.
*Posterior nucleus of the hypothalamus*
- The **posterior nucleus of the hypothalamus** is involved in **thermoregulation** and response to cold.
- It is not directly involved in the synthesis of **ADH**.
