MD Renal and Urinary Systems 3 — Questions and Answers
Question 1: A 70-year-old man cannot empty his bladder and has a post-void residual of 350 mL. PSA is elevated. What is the most likely cause?
- Detrusor overactivity
- Benign prostatic hyperplasia (Correct answer)
- Urethral stricture
- Neurogenic bladder
Correct answer: Benign prostatic hyperplasia
BPH causes bladder outlet obstruction in older men, leading to urinary retention and elevated post-void residual volumes.
Question 2: Renal tubular acidosis type IV is caused by which mechanism?
- Defective proximal HCO3 reabsorption
- Defective distal H+ secretion
- Hypoaldosteronism impairing K+ and H+ excretion (Correct answer)
- Carbonic anhydrase deficiency
Correct answer: Hypoaldosteronism impairing K+ and H+ excretion
Type IV RTA results from aldosterone deficiency or resistance, impairing both K+ and H+ secretion in the collecting duct, causing hyperkalemia and a normal anion gap acidosis.
Question 3: A patient on lithium therapy develops polyuria and polydipsia. Water deprivation test shows urine does not concentrate even after desmopressin. What is the diagnosis?
- Central diabetes insipidus
- Nephrogenic diabetes insipidus (Correct answer)
- Primary polydipsia
- SIADH
Correct answer: Nephrogenic diabetes insipidus
Lithium causes nephrogenic DI by impairing collecting duct response to ADH; urine fails to concentrate despite exogenous desmopressin.
Question 4: Which finding on urinalysis is most consistent with acute tubular necrosis?
- RBC casts
- WBC casts
- Muddy brown granular casts (Correct answer)
- Oval fat bodies
Correct answer: Muddy brown granular casts
ATN causes sloughing of tubular epithelial cells that form muddy brown granular casts, which are pathognomonic for this condition.
Question 5: A patient with HIV develops proteinuria and focal segmental glomerulosclerosis with collapsing variant. What is the most likely diagnosis?
- Lupus nephritis
- HIV-associated nephropathy (Correct answer)
- Diabetic nephropathy
- Amyloidosis
Correct answer: HIV-associated nephropathy
HIV-associated nephropathy (HIVAN) classically presents with FSGS collapsing variant, heavy proteinuria, and rapid progression to ESRD in HIV-positive patients.
Question 6: Which hormone directly stimulates the principal cells of the collecting duct to insert aquaporin-2 channels into the apical membrane?
- Aldosterone
- Antidiuretic hormone (ADH) (Correct answer)
- Atrial natriuretic peptide
- Parathyroid hormone
Correct answer: Antidiuretic hormone (ADH)
ADH (vasopressin) binds V2 receptors on principal cells, triggering cAMP-mediated insertion of aquaporin-2 into the apical membrane to increase water permeability.
Question 7: A 35-year-old woman with SLE has red cell casts and nephritic syndrome. Biopsy shows wire loop lesions and subendothelial deposits. What class of lupus nephritis is this?
- Class II (mesangial proliferative)
- Class III (focal proliferative)
- Class IV (diffuse proliferative) (Correct answer)
- Class V (membranous)
Correct answer: Class IV (diffuse proliferative)
Class IV lupus nephritis is the most severe form, characterized by diffuse proliferative changes with wire loop lesions from subendothelial immune complex deposits.
MD Renal and Urinary Systems 3
A 70-year-old man cannot empty his bladder and has a post-void residual of 350 mL.
PSA is elevated.
What is the most likely cause?