A 48-year-old man with a family history of kidney disease presents with hypertension and flank pain. Renal ultrasound shows bilaterally enlarged kidneys with the appearance shown in the diagram. The structure marked **A** (numerous cysts of varying sizes) is characteristic of ADPKD. Which of the following best explains the pathophysiology underlying cyst formation in this condition?

Question

Accepted Answer

D. Loss of polycystin-1 or polycystin-2 function in primary cilia, leading to dysregulation of intracellular calcium and abnormal tubular cell proliferation. ## Why option 1 is correct

The pathophysiology of ADPKD hinges on loss-of-function mutations in PKD1 (chromosome 16p13.3, encoding polycystin-1, ~78% of cases) or PKD2 (chromosome 4q21, encoding polycystin-2, ~15% of cases). Polycystins form a complex in the primary cilia of renal tubular epithelial cells that regulates intracellular calcium homeostasis and cell proliferation. When this complex is disrupted, intracellular calcium signaling is dysregulated, leading to uncontrolled tubular epithelial cell proliferation and cyst formation along the entire nephron. The structure marked **A** — innumerable cysts of varying sizes — is the direct morphological consequence of this polycystin dysfunction. This is the foundational molecular mechanism underlying ADPKD and is central to understanding disease pathogenesis and therapeutic targets (e.g., tolvaptan, a V2 receptor antagonist that suppresses cAMP-mediated proliferation).

## Why each distractor is wrong

- **Option 2**: UMOD gene mutations cause uromodulin-associated kidney disease (UAKD), a different monogenic disorder characterized by intratubular crystal deposition and progressive CKD, not cyst formation. This is not the mechanism of ADPKD.
- **Option 3**: Aquaporin-2 deficiency causes nephrogenic diabetes insipidus (NDI), characterized by polyuria and inability to concentrate urine, not cyst formation. It is not involved in ADPKD pathogenesis.
- **Option 4**: Aquaporin-1 overexpression is not a recognized mechanism in ADPKD. Aquaporins are water channels involved in osmotic balance, not in the calcium-proliferation dysregulation that drives cystogenesis.

**High-Yield:** ADPKD = polycystin dysfunction in primary cilia → loss of intracellular Ca²⁺ regulation → uncontrolled tubular proliferation → cyst formation; PKD1 (78%, earlier ESRD ~55y) vs PKD2 (15%, later ESRD ~75y).

[cite: KDIGO 2024 ADPKD Conference Update]

Answer

A. Mutation in the UMOD gene causing defective uromodulin production and intratubular crystal deposition

Answer

B. Deficiency of aquaporin-2 water channels resulting in impaired urine concentration and tubular dilation

Answer

C. Overexpression of aquaporin-1 in the collecting duct leading to excessive water reabsorption and cyst expansion

Explanation

Why option 1 is correct

Why each distractor is wrong

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