Question 1: Renal Cell Carcinoma (RCC)
Stem: A 65-year-old man undergoes a multiphase contrast-enhanced CT scan for haematuria. The scan reveals a 5 cm, exophytic mass arising from the right kidney. The mass is heterogeneous and demonstrates intense, avid enhancement on the corticomedullary phase, which is greater than the enhancement of the renal cortex. The enhancement then washes out on the nephrographic phase.
Question: This specific enhancement pattern is most characteristic of which subtype of RCC?
(A) Clear Cell RCC (B) Papillary RCC (C) Chromophobe RCC (D) Oncocytoma (E) Angiomyolipoma (AML)
Correct Answer: (A) Clear Cell RCC.
Explanation:
- Why (A) is correct: Clear Cell RCC is the most common subtype (~75%) and is typically hypervascular. Its hallmark is intense arterial/corticomedullary enhancement, often greater than the renal cortex, followed by washout. This hypervascularity is due to its genetic link (VHL gene) leading to high VEGF expression.
- Why (B) is wrong: Papillary RCC is typically hypovascular, showing minimal, gradual enhancement.
- Why (C) is wrong: Chromophobe RCC shows moderate, relatively homogeneous enhancement, less avid than clear cell, and often has a central scar (though less specific than oncocytoma).
- Why (D) is wrong: Oncocytoma is a benign tumour that can be hypervascular but classically has a central stellate scar and does not typically enhance more avidly than the cortex. Washout is less common.
- Why (E) is wrong: An AML is diagnosed by the presence of macroscopic fat (-10 HU or less) within the lesion on non-contrast CT. If it’s fat-poor, it can mimic RCC, but the intense enhancement described is most typical for clear cell.
Key Points: Clear Cell RCC
- Most common subtype of RCC.
- Hallmark: Hypervascular (intense arterial/corticomedullary enhancement, often > cortex) with washout.
- Often large, exophytic, heterogeneous (necrosis/cystic change).
- Associated with Von Hippel-Lindau (VHL) syndrome.
Question 2: Angiomyolipoma (AML)
Stem: A 45-year-old woman has an incidental finding on a non-contrast CT abdomen performed for flank pain. There is a 3 cm, well-defined, cortically-based lesion in the left kidney. The lesion is heterogeneous, but several voxels within it measure -40 Hounsfield Units (HU).
Question: What is the most likely diagnosis?
(A) Clear Cell RCC (B) Oncocytoma (C) Simple Renal Cyst (D) Angiomyolipoma (AML) (E) Haemorrhagic Cyst
Correct Answer: (D) Angiomyolipoma (AML).
Explanation:
- Why (D) is correct: The presence of macroscopic fat (attenuation values less than -10 HU, typically -20 to -100 HU) within a renal mass on non-contrast CT is pathognomonic for an Angiomyolipoma (AML). AMLs are benign tumours composed of blood vessels (angio-), smooth muscle (-myo-), and fat (-lipoma).
- Why (A) is wrong: RCC does not contain macroscopic fat (unless it engulfs sinus fat).
- Why (B) is wrong: Oncocytoma does not contain fat.
- Why (C) is wrong: A simple cyst is homogeneous water attenuation (0-20 HU).
- Why (E) is wrong: A haemorrhagic cyst would be high attenuation (50-70 HU) on non-contrast CT, not fat attenuation.
Key Points: Angiomyolipoma (AML)
- Definition: Benign renal tumour (part of the PEComa family).
- Pathognomonic Sign: Presence of macroscopic fat (< -10 HU) on non-contrast CT.
- Association: Strongly associated with Tuberous Sclerosis Complex (TSC) (multiple, bilateral AMLs).
- Risk: Can cause life-threatening spontaneous haemorrhage, especially if > 4 cm or aneurysmal vessels are present.
- Note: “Fat-poor” AMLs exist and are difficult to distinguish from RCC on imaging alone.
Question 3: Renal Cyst (Bosniak Classification)
Stem: A 60-year-old man has a complex cystic renal lesion identified on a contrast-enhanced CT. The lesion measures 5 cm, contains multiple, thin (<1 mm) septations, and shows minimal, non-measurable enhancement of these septations. No solid components are seen.
Question: According to the Bosniak classification (v2019), what category does this lesion fall into?
(A) Bosniak I (B) Bosniak II (C) Bosniak IIF (D) Bosniak III (E) Bosniak IV
Correct Answer: (B) Bosniak II.
Explanation:
- Why (B) is correct: A Bosniak II cyst is considered benign. It includes cysts with few (<4), thin (<1-2 mm) septations that show no measurable enhancement. Minimally complex cysts that do not require follow-up fall into this category. (Note: v2019 simplified things; previously, minimal enhancement might push it to IIF).
- Why (A) is wrong: Bosniak I is a simple cyst (thin wall, no septa, no calcification, no enhancement).
- Why (C) is wrong: Bosniak IIF (“F” for follow-up) includes cysts with more numerous or slightly thicker septa, or minimally thickened wall/septa, or perceived (but non-measurable) enhancement. These require surveillance.
- Why (D) is wrong: Bosniak III includes cysts with thickened, irregular, or measurably enhancing septa or walls. These are indeterminate and often require surgery.
- Why (E) is wrong: Bosniak IV includes cysts with definite solid, enhancing nodular components. These are considered malignant until proven otherwise.
Key Points: Bosniak Classification (v2019 Simplified)
- Used to stratify the malignancy risk of cystic renal masses.
- I: Simple Cyst (Benign).
- II: Minimally Complex (few thin septa, non-enhancing). (Benign).
- IIF: Requires Follow-up (more complex septa/wall, perceived enhancement). (Low malignancy risk).
- III: Indeterminate (thick/enhancing septa/wall). (Moderate malignancy risk).
- IV: Malignant (solid enhancing components). (High malignancy risk).
Question 4: Adrenal Adenoma
Stem: A 55-year-old woman has an incidental 2 cm left adrenal nodule found on a non-contrast CT abdomen. The mean attenuation value of the nodule is measured at 5 Hounsfield Units (HU).
Question: This attenuation value strongly suggests the nodule is a benign:
(A) Adrenal Adenoma (B) Phaeochromocytoma (C) Adrenal Carcinoma (D) Adrenal Metastasis (E) Adrenal Cyst
Correct Answer: (A) Adrenal Adenoma.
Explanation:
- Why (A) is correct: The key diagnostic feature of a benign adrenal adenoma is the presence of intracytoplasmic lipid. Fat is low density. On non-contrast CT, an adenoma is diagnosed if the attenuation is ≤ 10 HU. This finding is highly specific for a benign, lipid-rich adenoma.
- Why (B), (C), (D) are wrong: Phaeochromocytomas, adrenal carcinomas, and metastases are typically lipid-poor. They will almost always measure > 10 HU (often 20-40 HU) on non-contrast CT.
- Why (E) is wrong: An adrenal cyst would be simple fluid attenuation (0-20 HU, similar range but typically lower end) and would have a thin, imperceptible wall, unlike a solid adenoma. However, ≤10 HU strongly favours adenoma.
Key Points: Adrenal Adenoma Characterization (CT)
- Non-Contrast CT:
- Diagnosis: Attenuation ≤ 10 HU = Lipid-rich adenoma (Benign).
- Contrast-Enhanced CT (Washout Protocol – for >10 HU lesions):
- Adenomas enhance and then rapidly wash out contrast due to their lipid content.
- Absolute Washout > 60% = Adenoma.
- Relative Washout > 40% = Adenoma.
- Note: Malignant lesions (mets, carcinoma, phaeo) are lipid-poor and show delayed washout.
Question 5: Phaeochromocytoma
Stem: A 40-year-old man presents with episodic severe headaches, palpitations, and sweating. His blood pressure is markedly elevated during these episodes. An MRI of the abdomen is performed. It reveals a 5 cm, well-defined right adrenal mass that is markedly hyperintense (bright) on T2-weighted images, resembling a “lightbulb.”
Question: This “lightbulb” sign on T2-weighted MRI is highly characteristic of:
(A) Adrenal Adenoma (B) Phaeochromocytoma (C) Adrenal Carcinoma (D) Adrenal Metastasis (E) Myelolipoma
Correct Answer: (B) Phaeochromocytoma.
Explanation:
- Why (B) is correct: Phaeochromocytomas are tumours of the adrenal medulla that secrete catecholamines. While their CT appearance can be variable (often large, cystic, necrotic), the marked T2 hyperintensity (“lightbulb” sign) on MRI is a highly characteristic feature, seen in about 65% of cases.
- Why (A) is wrong: Adenomas are typically isointense or mildly hyperintense on T2, not “lightbulb” bright.
- Why (C) is wrong: Adrenal carcinoma is usually large, heterogeneous, and invasive, without marked T2 hyperintensity.
- Why (D) is wrong: Metastases are variable but not typically this T2 bright.
- Why (E) is wrong: A myelolipoma is a benign tumour containing macroscopic fat (T1 bright, signal loss on fat-sat).
Key Points: Phaeochromocytoma
- Definition: Catecholamine-secreting tumour of the adrenal medulla (or extra-adrenal paraganglia).
- “Rule of 10s”: 10% bilateral, 10% extra-adrenal, 10% malignant, 10% paediatric, 10% familial.
- Clinical: Paroxysmal hypertension, headache, palpitations, sweating.
- Imaging:
- CT: Often large, heterogeneous, cystic/necrotic. Avidly enhances.
- MRI: “Lightbulb” sign (marked T2 hyperintensity) is characteristic.
- MIBG Scan: Specific nuclear medicine scan for functional tissue.
Question 6: Adrenal Myelolipoma
Stem: A 60-year-old woman has an incidental 4 cm left adrenal mass found on CT. The mass is well-defined and contains large areas of macroscopic fat (measuring -70 HU), interspersed with soft-tissue density elements.
Question: What is the most likely diagnosis?
(A) Adrenal Adenoma (B) Phaeochromocytoma (C) Adrenal Carcinoma (D) Adrenal Metastasis (E) Myelolipoma
Correct Answer: (E) Myelolipoma.
Explanation:
- Why (E) is correct: The presence of macroscopic fat within an adrenal mass is essentially diagnostic of a myelolipoma. This is a rare, benign tumour composed of mature adipose tissue (lipo-) and haematopoietic elements (-myelo-).
- Why (A) is wrong: Adenomas contain microscopic (intracytoplasmic) lipid, which makes them low density (≤ 10 HU) but not usually macroscopic fat density (<-10 HU).
- Why (B), (C), (D) are wrong: Phaeochromocytomas, carcinomas, and metastases do not contain macroscopic fat.
Key Points: Adrenal Myelolipoma
- Definition: Rare, benign tumour of fat and haematopoietic tissue.
- Imaging (Pathognomonic): Presence of macroscopic fat within an adrenal mass on CT or MRI.
- Clinical: Usually asymptomatic and incidental. Can rarely haemorrhage if large.
Question 7: Renal Artery Stenosis (Atherosclerosis)
Stem: A 75-year-old man with poorly controlled hypertension and peripheral vascular disease undergoes a CT angiogram for investigation of renal artery stenosis. The scan reveals a high-grade, calcified stenosis located within the first 1 cm of the origin of the right renal artery.
Question: This location is most typical for renal artery stenosis caused by:
(A) Atherosclerosis (B) Fibromuscular Dysplasia (FMD) (C) Takayasu’s Arteritis (D) Median Arcuate Ligament Syndrome (E) Renal Artery Aneurysm
Correct Answer: (A) Atherosclerosis.
Explanation:
- Why (A) is correct: Atherosclerosis is the most common cause of renal artery stenosis overall (~90%). It classically affects older patients with other vascular risk factors and involves the ostium (origin) or proximal segment of the renal artery.
- Why (B) is wrong: FMD is the most common cause in younger patients (especially women) and classically affects the mid-to-distal segments, often with a “string of beads” appearance.
- Why (C) is wrong: Takayasu’s is a large-vessel vasculitis causing long-segment wall thickening, typically in younger women.
- Why (D) is wrong: This involves compression of the coeliac artery, not the renal artery.
- Why (E) is wrong: An aneurysm is a dilatation, not a stenosis.
Key Points: Renal Artery Stenosis
- Atherosclerosis (Most Common):
- Location: Ostial / Proximal.
- Demographic: Older patients, vascular risk factors.
- Fibromuscular Dysplasia (FMD):
- Location: Mid / Distal.
- Appearance: “String of beads”.
- Demographic: Younger females.
Question 8: Transitional Cell Carcinoma (TCC)
Stem: A 70-year-old heavy smoker presents with painless macroscopic haematuria. A CT urogram is performed. On the delayed (excretory) phase images, a 2 cm, irregular, sessile filling defect is seen arising from the wall of the renal pelvis.
Question: What is the most likely diagnosis?
(A) Transitional Cell Carcinoma (TCC) (B) Renal Cell Carcinoma (RCC) (C) Renal Calculus (D) Blood Clot (E) Pyelonephritis
Correct Answer: (A) Transitional Cell Carcinoma (TCC).
Explanation:
- Why (A) is correct: TCC (also called Urothelial Carcinoma) arises from the urothelium lining the collecting system (renal pelvis, ureter, bladder). The classic appearance on excretory phase imaging (CTU or IVU) is an irregular filling defect within the contrast-filled collecting system. Smoking is a major risk factor.
- Why (B) is wrong: RCC is a tumour of the renal parenchyma (cortex), not the collecting system lining.
- Why (C) is wrong: A calculus (stone) would be hyperdense on non-contrast CT and is typically mobile (unless impacted).
- Why (D) is wrong: A blood clot can also cause a filling defect but is typically smooth, mobile, and resolves on follow-up. TCC is fixed and irregular.
- Why (E) is wrong: Pyelonephritis is an infection of the parenchyma, causing striated nephrograms or abscesses, not a filling defect in the pelvis.
Key Points: Transitional Cell Carcinoma (TCC/UCC)
- Definition: Malignancy of the urothelium.
- Locations: Bladder (most common) > Renal Pelvis > Ureter.
- Risk Factors: Smoking, industrial carcinogens (aniline dyes), cyclophosphamide.
- Imaging (CT Urogram):
- Excretory Phase: Irregular filling defect in the contrast-filled collecting system.
- Nephrographic Phase: May show focal wall thickening or enhancement.
- Note: Often multifocal (“field change”). Look for synchronous or metachronous tumours elsewhere in the urothelium.
Question 9: Acute Pyelonephritis
Stem: A 30-year-old woman presents with high fever, flank pain, and dysuria. A contrast-enhanced CT scan performed during the nephrographic phase shows multiple, wedge-shaped areas of decreased enhancement extending from the papilla to the cortex in the right kidney. The kidney is slightly enlarged.
Question: This appearance is characteristic of:
(A) Acute Pyelonephritis (B) Renal Infarcts (C) Renal Lymphoma (D) Polycystic Kidney Disease (E) Renal Cell Carcinoma
Correct Answer: (A) Acute Pyelonephritis.
Explanation:
- Why (A) is correct: This is the classic CT appearance of acute bacterial pyelonephritis (kidney infection). The infection causes inflammation and oedema, which impairs perfusion and contrast uptake. This results in striated or wedge-shaped regions of poor enhancement (a “striated nephrogram”).
- Why (B) is wrong: Renal infarcts are also wedge-shaped areas of non-enhancement, but they are typically sharply demarcated, associated with a vascular cause (e.g., embolus), and the kidney is usually not enlarged. Pyelonephritis has fuzzier margins.
- Why (C) is wrong: Renal lymphoma typically presents as multiple, bilateral, hypovascular masses or diffuse infiltration, not striated non-enhancement.
- Why (D) is wrong: This involves multiple simple cysts throughout the kidneys.
- Why (E) is wrong: RCC is a focal, enhancing mass.
Key Points: Acute Pyelonephritis
- Definition: Bacterial infection of the renal parenchyma and pelvis.
- Clinical: Fever, flank pain, pyuria.
- CT Findings (Nephrographic Phase):
- Striated Nephrogram: Wedge-shaped or linear areas of decreased enhancement.
- Often associated with renal enlargement and perinephric fat stranding.
- Complications: Renal abscess, pyonephrosis (pus in obstructed system), emphysematous pyelonephritis (gas).
Question 10: Emphysematous Pyelonephritis
Stem: A 65-year-old poorly controlled diabetic woman presents with septic shock and flank pain. A CT scan reveals an enlarged, inflamed right kidney containing multiple foci of gas within the renal parenchyma. Gas is also seen tracking into the perinephric space.
Question: What is this life-threatening condition?
(A) Xanthogranulomatous Pyelonephritis (XGP) (B) Pyonephrosis (C) Emphysematous Pyelonephritis (D) Renal Abscess (E) Renal Infarct with secondary infection
Correct Answer: (C) Emphysematous Pyelonephritis.
Explanation:
- Why (C) is correct: Gas within the renal parenchyma is the defining feature of emphysematous pyelonephritis, a severe, necrotising infection caused by gas-forming organisms (e.g., E. coli, Klebsiella). It is strongly associated with diabetes mellitus and carries a high mortality rate.
- Why (A) is wrong: XGP is a chronic destructive process (often related to staghorn calculi) where the kidney is replaced by lipid-laden macrophages, appearing as a non-functioning, enlarged kidney with low-density areas (fat), but not acute gas.
- Why (B) is wrong: Pyonephrosis is pus within an obstructed collecting system, which appears as high-density fluid, not parenchymal gas.
- Why (D) is wrong: A renal abscess is a collection of pus, which might contain gas bubbles, but emphysematous pyelonephritis is a diffuse gas-forming parenchymal infection.
- Why (E) is wrong: While possible, primary emphysematous pyelonephritis is the classic diagnosis for parenchymal gas in a diabetic.
Key Points: Emphysematous Pyelonephritis
- Definition: A life-threatening, necrotising, gas-forming infection of the renal parenchyma.
- Risk Factors: Diabetes Mellitus (90%), immunosuppression, obstruction.
- CT Finding (Pathognomonic): Gas within the renal parenchyma, perinephric space, or collecting system.
- Management: Often requires urgent nephrectomy.
Question 11: Xanthogranulomatous Pyelonephritis (XGP)
Stem: A 55-year-old woman with recurrent urinary tract infections presents with flank pain. A CT scan shows an enlarged, non-functioning left kidney containing a large staghorn calculus filling the renal pelvis. The renal parenchyma is replaced by multiple, low-density, non-enhancing collections. The central low-density areas have attenuation values suggestive of fat.
Question: This appearance is most characteristic of:
(A) Xanthogranulomatous Pyelonephritis (XGP) (B) Emphysematous Pyelonephritis (C) Renal Tuberculosis (D) Polycystic Kidney Disease (E) Multilocular Cystic Nephroma
Correct Answer: (A) Xanthogranulomatous Pyelonephritis (XGP).
Explanation:
- Why (A) is correct: This is the classic XGP presentation. It is a rare, chronic, destructive granulomatous process that mimics a tumour (“-oma”). Key features are: 1) Unilateral, enlarged, non-functioning kidney, 2) A large staghorn calculus (present in ~80%), and 3) Replacement of parenchyma by low-density (lipid-laden macrophage) collections, often described as a “bear paw” sign. The presence of fat density is highly suggestive.
- Why (B) is wrong: This is an acute infection with gas, not a chronic process with fat.
- Why (C) is wrong: Renal TB causes scarring, calcification (“putty kidney”), and strictures, not this mass-like appearance with fat.
- Why (D) is wrong: PCKD involves bilateral, simple cysts.
- Why (E) is wrong: This is a rare, benign cystic neoplasm, not an inflammatory process related to stones.
Key Points: Xanthogranulomatous Pyelonephritis (XGP)
- Definition: Chronic, destructive granulomatous inflammation (lipid-laden macrophages).
- Associations: Staghorn calculus and chronic obstruction/infection.
- Imaging:
- Enlarged, non-functioning kidney (unilateral).
- Staghorn calculus.
- Parenchyma replaced by low-density collections (may contain fat density).
- “Bear Paw Sign”: Dilated calyces filled with low-density material surrounding a contracted pelvis.
Question 12: Renal Trauma (AAST Grade)
Stem: A 28-year-old man sustains blunt abdominal trauma in a fall. A contrast-enhanced CT shows a deep cortical laceration in the right kidney extending into the collecting system, causing urinary extravasation. There is also a small focus of active arterial contrast extravasation.
Question: According to the AAST renal injury grading scale, what is the minimum grade for this injury?
(A) Grade II (B) Grade III (C) Grade IV (D) Grade V (E) Grade I
Correct Answer: (C) Grade IV.
Explanation:
- Why (C) is correct: The presence of urinary extravasation (indicating injury to the collecting system) OR active arterial bleeding (contrast extravasation) automatically places the injury into Grade IV according to the AAST scale. Grade IV also includes injuries involving the main renal artery or vein with contained haemorrhage, or segmental infarcts.
- Why (A) & (B) are wrong: Grades II and III involve progressively deeper lacerations without collecting system injury or active bleeding.
- Grade II: Laceration < 1 cm deep, no extravasation.
- Grade III: Laceration > 1 cm deep, no collecting system injury.
- Why (D) is wrong: Grade V involves a shattered kidney or avulsion of the renal hilum (pedicle injury).
- Why (E) is wrong: Grade I is a contusion or subcapsular haematoma only.
Key Points: AAST Renal Injury Scale (Simplified)
- Grade I: Contusion/Subcapsular haematoma.
- Grade II: Laceration < 1 cm deep (cortex only).
- Grade III: Laceration > 1 cm deep (into medulla).
- Grade IV: Laceration into collecting system (urine leak) OR vascular injury (arterial bleed, segmental infarct).
- Grade V: Shattered kidney OR pedicle avulsion.
Question 13: Conn’s Syndrome
Stem: A 45-year-old woman presents with treatment-resistant hypertension and hypokalaemia. A CT scan reveals a 1.2 cm, well-defined, lipid-rich (< 10 HU on non-contrast) nodule in the left adrenal gland.
Question: This clinical and radiological picture is most consistent with:
(A) Conn’s Syndrome (Aldosteronoma) (B) Cushing’s Syndrome (Cortisol-producing adenoma) (C) Phaeochromocytoma (D) Non-functioning Adrenal Adenoma (E) Adrenal Metastasis
Correct Answer: (A) Conn’s Syndrome (Aldosteronoma).
Explanation:
- Why (A) is correct: Conn’s syndrome is primary hyperaldosteronism, most commonly caused by a small, benign aldosterone-producing adenoma. The clinical hallmarks are hypertension and hypokalaemia. Radiologically, these adenomas are typically small (<2 cm) and lipid-rich (≤ 10 HU), identical in appearance to non-functioning adenomas. The clinical context is key.
- Why (B) is wrong: Cushing’s syndrome (hypercortisolism) presents with central obesity, moon facies, striae, etc. The adenoma looks identical, but the symptoms differ.
- Why (C) is wrong: Phaeochromocytomas cause paroxysmal hypertension and are usually lipid-poor (>10 HU).
- Why (D) is wrong: This is possible based on imaging alone, but the clinical symptoms (hypertension + hypokalaemia) point strongly to Conn’s.
- Why (E) is wrong: Metastases are typically lipid-poor (>10 HU).
Key Points: Functioning Adrenal Adenomas
- Imaging Appearance: Usually identical to non-functioning adenomas (small, lipid-rich). Diagnosis is clinical/biochemical.
- Conn’s Syndrome: Aldosterone excess -> Hypertension + Hypokalaemia.
- Cushing’s Syndrome: Cortisol excess -> Weight gain, striae, moon facies.
- Note: Adrenal Venous Sampling may be needed to lateralise the source of hormone excess before surgery.
Question 14: Adrenal Haemorrhage
Stem: A 60-year-old man on warfarin for atrial fibrillation develops acute flank pain and hypotension. A non-contrast CT scan shows bilateral enlargement of the adrenal glands, which appear hyperdense (mean attenuation 65 HU) and have surrounding inflammatory fat stranding.
Question: What is the most likely diagnosis?
(A) Bilateral Adrenal Metastases (B) Bilateral Phaeochromocytomas (C) Bilateral Adrenal Haemorrhage (D) Bilateral Adrenal Adenomas (E) Adrenal Lymphoma
Correct Answer: (C) Bilateral Adrenal Haemorrhage.
Explanation:
- Why (C) is correct: The key finding is the high attenuation (50-90 HU) of the enlarged adrenal glands on non-contrast CT, which represents acute blood. Bilateral adrenal haemorrhage is a known complication in anticoagulated patients or patients under severe stress (e.g., sepsis). It can lead to acute adrenal insufficiency (Addisonian crisis).
- Why (A) is wrong: Metastases would be soft-tissue density (30-50 HU) and would enhance.
- Why (B) is wrong: Phaeochromocytomas are soft-tissue density and enhance avidly.
- Why (D) is wrong: Adenomas are typically low density (≤ 10 HU).
- Why (E) is wrong: Lymphoma is rare and would be soft-tissue density.
Key Points: Adrenal Haemorrhage
- Causes: Anticoagulation, trauma, severe stress (sepsis, surgery, burns), neonatal hypoxia.
- CT (Non-contrast):
- Acute: Gland enlargement, hyperdense (50-90 HU), surrounding stranding.
- Subacute/Chronic: Becomes hypodense, cystic, may calcify.
- Clinical: Can present with flank pain, hypotension, and acute adrenal insufficiency.
Question 15: Horseshoe Kidney
Stem: A 40-year-old man has an incidental finding on an abdominal CT. The lower poles of both kidneys are fused anterior to the aorta and IVC by a band of functioning renal parenchyma. The renal pelves and ureters are noted to be anteriorly rotated.
Question: What is this common congenital anomaly?
(A) Crossed Fused Renal Ectopia (B) Pelvic Kidney (C) Duplex Kidney (D) Horseshoe Kidney (E) Renal Agenesis
Correct Answer: (D) Horseshoe Kidney.
Explanation:
- Why (D) is correct: This is the definition of a horseshoe kidney, the most common renal fusion anomaly. The lower poles are fused by an isthmus (usually parenchyma, sometimes fibrous) that lies anterior to the great vessels. This fusion prevents normal ascent and rotation, leading to the low position and anteriorly directed renal pelves.
- Why (A) is wrong: In crossed fused ectopia, both kidneys are on the same side of the body and are fused together.
- Why (B) is wrong: A pelvic kidney is a kidney that fails to ascend and remains in the pelvis, but it is not fused to the contralateral kidney.
- Why (C) is wrong: A duplex kidney has two separate collecting systems, which may or may not be fused.
- Why (E) is wrong: This is the absence of a kidney.
Key Points: Horseshoe Kidney
- Most common renal fusion anomaly (1 in 400).
- Fusion: Lower poles are fused by an isthmus anterior to the aorta/IVC.
- Position: Kidneys are low-lying (ascent arrested by the IMA) and malrotated (pelves face anteriorly).
- Complications: Increased risk of UPJ obstruction, stones, infection, trauma, and certain tumours (Wilms’, carcinoid).
