WBR0104
| Author | [[PageAuthor::William J Gibson (reviewed by Rim Halaby, M.D. [1])]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Genetics |
| Sub Category | SubCategory::Renal |
| Prompt | [[Prompt::A 40 year old woman with a history of recurrent urinary tract infections presents to her primary care physician with the gradual onset of increasing fatigue, swelling in her feet, and anorexia. She reports flank pain, and a recent decrease in urine volume. On examination, her vital signs are: blood pressure of 145/110, pulse rate of 76 beats/min, and temperature of 37.2C (98.9 F). Physical exam is remarkable for bilateral solid masses that can be palpated during abdominal examination. Tapping the lower back elicits sharp pain. Laboratory studies are significant for a creatinine of 2.3 mg/dL, and BUN of 20 mmol/L. Which of the following is most likely true regarding this patient’s condition?]] |
| Answer A | AnswerA::Caused by a defect in a calcium channel gene |
| Answer A Explanation | AnswerAExp::ADPKD2 is caused by defects in a calcium channel gene. ADPKD2 has a later onset and is rarer than ADPKD1 mutations. |
| Answer B | AnswerB::Caused by a defect in the assembly of the cilium |
| Answer B Explanation | [[AnswerBExp::This patient's form of autosomal dominant polycystic kidney disease is caused by ADPKD1 mutations. ADPKD1 is thought to encode a cellular membrane protein involved in assembly of the cilium.]] |
| Answer C | AnswerC::Caused by autoantibodies against glomerular basement membrane |
| Answer C Explanation | [[AnswerCExp::Goodpasture's disease is caused by autoantibodies against glomerular basement membrane which would not cause renal masses.]] |
| Answer D | AnswerD::Transmitted in an autosomal recessive fashion |
| Answer D Explanation | [[AnswerDExp::Autosomal dominant polycystic kidney disease follows an autosomal dominant pattern of inheritance. There is an autosomal recessive variant of polycystic kidney disease, but this variant leads to very severe early presenting disease. Enlarged kidneys can be detected at birth and 50% of neonates die of pulmonary hypoplasia due to oligohydramnios from renal failure. It is far more likely that the patient in this vignette has ADPKD type I.]] |
| Answer E | AnswerE::Associated with abdominal aortic aneurysms |
| Answer E Explanation | [[AnswerEExp::While a relationship between ADPKD and abdominal aortic aneurysms has been proposed, a study of 139 patients failed to detect any increased risk of abdominal aortic aneurysms (Torra et al). Instead, ADPKD is associated with cerebral berry aneurysms. The incidence of these aneurysms climbs to greater than 20 percent for patients aged 60 years and above.]] |
| Right Answer | RightAnswer::B |
| Explanation | [[Explanation::The onset of symptoms suggesting renal failure and bilateral abdominal masses strongly suggests the diagnosis of autosomal dominant polycystic kidney disease in this patient. Approximately 50% of people with this disease will develop end stage kidney disease and require dialysis or kidney transplantation. Progression to end stage kidney disease usually happens in the 4th to 6th decades of life. Autosomal dominant polycystic kidney disease affects about 1 in 400 to 1 in 1000 people.
Defects in two genes are thought to be responsible for ADPKD. In 85% of patients, ADPKD is caused by mutations in the gene PKD1 on chromosome 16 (TRPP1); in 15% of patients mutations in PKD2 (TRPP2) are causative. The median age at which patients develop renal failure is 54 for PKD1 patients and 78 for PKD2 patients. Because PKD1 is more common generally, and this woman is presenting with signs of renal failure at age 40, it is far more likely that she has PKD1 than PKD2. The defective gene in ADPKD1 is thought to encode a cellular membrane protein involved in assembly of the cilium. While there are no FDA approved drugs for PKD, recent studies have showed that the vasopressin antagonist tolvaptan is able to slow the progression of the disease. Patients are often initiated on ACE inhibitors to decrease hypertension resulting from renal dysfunction. Patients who have developed renal failure must undergo hemodialysis or kidney transplantation. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::ADPKD, WBRKeyword::Autosomal dominant, WBRKeyword::Kidney, WBRKeyword::Renal, WBRKeyword::Polycystic kidney disease |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |