A neonate has bilateral flank masses, oligohydramnios was noted antenatally, and the baby shows Potter facies with deformed ears and clubfoot. Ultrasound shows absent kidneys and severe pulmonary hypoplasia. The primary embryological failure is:

Option D is correct because bilateral renal agenesis results from failure of the ureteric bud to induce the metanephric blastema, the reciprocal interaction that generates the permanent kidney.

Why failure of ureteric bud induction causes bilateral renal agenesis

The permanent kidney, the metanephros, arises from two embryological components that must interact. The ureteric bud is an epithelial outgrowth from the caudal end of the mesonephric (Wolffian) duct. It grows dorsally and cranially into the intermediate mesoderm, where it contacts the metanephric blastema, a specialised condensation of metanephric mesenchyme. The ureteric bud induces the metanephric blastema to condense, proliferate, and undergo mesenchymal-to-epithelial transformation to form the nephron precursors: glomeruli, proximal and distal convoluted tubules, and loops of Henle. Simultaneously, the metanephric blastema induces the ureteric bud to branch repeatedly, forming the collecting duct system, minor calyces, major calyces, renal pelvis, and ureter. This reciprocal induction is governed by GDNF-RET signalling from the blastema to the bud, and by FGF, Wnt, and BMP pathways from the bud to the blastema. If the ureteric bud fails to form, fails to reach the blastema, or fails to deliver inductive signals, the metanephric blastema undergoes apoptosis rather than differentiation. The result is complete absence of renal tissue on both sides. Bilateral renal agenesis is therefore fundamentally a failure of ureteric bud induction of the metanephric blastema, which is option D. The mesonephric duct itself may also fail to form in some cases, but the proximate embryological event tested here is the bud-blastema interaction.

From absent kidneys to Potter sequence: the causal chain

The clinical findings in the stem form a single pathophysiological cascade. Bilateral renal agenesis means no fetal urine production from approximately week 10 onward. Amniotic fluid volume after the first trimester is maintained almost entirely by fetal urine, with a minor contribution from pulmonary fluid. Absent urine output produces severe oligohydramnios. The oligohydramnios is the central lesion from which every other feature follows. The amnion is mechanically compressed against the fetal skin. The fetus cannot move freely, producing positional deformities: clubfoot (talipes equinovarus), contractures of the limbs, and joint rigidity. The ears are low-set and flattened because the auricles, which normally elevate and rotate during the second and third trimesters, are compressed against the side of the head. The nose is compressed and broadened. The chin is receding. The skin is redundant and lax. Together these features constitute Potter facies, named after Edith Potter who described the association in 1946. The most lethal consequence is pulmonary hypoplasia. Normal lung development requires fetal breathing movements of amniotic fluid, which stretch the airway epithelium and stimulate branching morphogenesis. Severe oligohydramnios abolishes this mechanical stimulus, resulting in reduced airway branching, decreased alveolar number, and hypoplastic lungs that cannot sustain gas exchange after birth. Death occurs within hours from respiratory failure. Bilateral flank masses in this context are not enlarged kidneys but rather adrenal glands, which remain flattened and disc-shaped in the absence of renal ascent, and may be palpable as bilateral firm masses in the renal fossae. This finding is sometimes called the 'lying down' adrenal sign on imaging.

Why the other options do not produce this picture

Each distractor describes a real embryological event but produces a distinct clinical syndrome, none of which match the stem. Option A, abnormal division of the cloaca by the urorectal septum, produces anorectal malformations such as imperforate anourethral membrane or rectourethral fistula. The urorectal septum grows caudally to divide the cloaca into the urogenital sinus ventrally and the anorectal canal dorsally. Defective descent or fusion of this septum traps the rectum above, below, or at the level of the puborectalis muscle. The kidneys are unaffected because the metanephros and ureteric bud develop rostral to the cloaca and are not dependent on urorectal septation. Option B, failure of the pronephros to regress, is not a recognised cause of any human malformation. The pronephros is a vestigial structure in humans; its tubules degenerate by week 4 and its duct becomes the mesonephric duct. Persistence of pronephric tubules would not prevent metanephric development, because the metanephros arises independently from a new ureteric bud and new blastema. Option C, failure of fusion of the metanephric blastema across the midline, describes the embryology of horseshoe kidney. In horseshoe kidney, the inferior poles of the two metanephric blastemas fuse in the midline, and the resulting kidney is trapped under the inferior meseric artery during ascent. The patient has a single fused kidney, not absent kidneys, and urine production is preserved, so oligohydramnios and Potter sequence do not occur.

How this concept appears in postgraduate medical entrance exams

Bilateral renal agenesis and Potter sequence are high yield because they test embryology, clinical reasoning, and pathophysiology in a single stem. The exam most frequently asks for the embryological cause of bilateral renal agenesis, as in this question. A second common variant asks which structure is responsible for the flank masses in a neonate with absent kidneys, the answer being the adrenal gland. A third variant presents Potter facies and asks which prenatal finding explains it, the answer being oligohydramnios. Students must recognise that unilateral renal agenesis is compatible with normal life and does not produce Potter sequence, because the contralateral kidney compensates and urine output remains adequate. Only bilateral disease produces the full sequence. Another tested association is the link between oligohydramnios and pulmonary hypoplasia, which is mechanical rather than genetic. The exam may also ask about the signalling molecule most responsible for ureteric bud outgrowth from the mesonephric duct, the answer being GDNF (glial cell line derived neurotrophic factor) acting through the RET receptor on the bud tip. Mutations in GDNF or RET produce bilateral renal agenesis in animal models and are implicated in some human cases. In syndromic contexts, bilateral renal agenesis can occur as part of Fraser syndrome (cryptophthalmos syndrome), which features renal agenesis, syndactyly, genital anomalies, and cryptophthalmos, and is autosomal recessive.

Why the other options fail

One-glance recall table

Mnemonics

What the exam actually asks

• Bilateral renal agenesis is the only renal condition that produces Potter sequence; unilateral agenesis does not.
• The flank masses in a neonate with bilateral renal agenesis are adrenal glands, not kidneys. This is a separate standalone question.
• GDNF from the metanephric blastema acting on RET receptors on the ureteric bud tip is the key signalling pathway tested for bud outgrowth.
• Fraser syndrome includes bilateral renal agenesis, syndactyly, and cryptophthalmos, and is autosomal recessive.
• Amniotic fluid after the first trimester is predominantly fetal urine, so absent kidneys produce oligohydramnios from approximately week 12 onward.

Traps that cost marks

• Choosing option A because the urorectal septum is part of urogenital embryology, without recognising that it produces anorectal malformations, not renal agenesis.
• Choosing option C because horseshoe kidney involves the metanephric blastema, without recognising that horseshoe kidney is a fusion anomaly with preserved renal function, not bilateral absence.
• Assuming that any renal anomaly produces Potter sequence, when only bilateral disease does so.
• Confusing Potter sequence (oligohydramnios sequence) with prune belly syndrome, which involves abdominal wall muscle deficiency, undescended testes, and urinary tract dilatation but not absent kidneys.

Frequently asked

Why are the flank masses in bilateral renal agenesis not kidneys?

The bilateral flank masses are adrenal glands. In the absence of kidneys, the adrenal glands remain in their fetal position, flattened and disc-shaped against the posterior abdominal wall, rather than becoming the small triangular glands that sit atop the kidneys. This produces the 'lying down' adrenal sign on ultrasound or CT. The adrenals are not dependent on the kidneys for their own development, so they are present even when the metanephros fails entirely.

What is the difference between Potter sequence and prune belly syndrome?

Potter sequence is caused by severe oligohydramnios, most commonly from bilateral renal agenesis or posterior urethral valves. It features Potter facies, limb deformities, and pulmonary hypoplasia. Prune belly syndrome is characterised by deficiency of the abdominal wall musculature, bilateral undescended testes, and dilatation of the urinary tract, but the kidneys are present and usually functional. The two conditions are embryologically distinct and should not be confused.

Can bilateral renal agenesis be diagnosed on prenatal ultrasound?

Yes. The prenatal ultrasound shows absent kidneys in the renal fossae, absent bladder filling, severe oligohydramnios, and the flattened 'lying down' adrenal glands. The diagnosis is usually made in the second trimester when oligohydramnios becomes apparent. Amnioinfusion can be performed to improve imaging and confirm absent renal tissue. The prognosis is uniformly lethal due to pulmonary hypoplasia.

References

• Langman's Medical Embryology, 14th. Chapter 16, the urogenital system, covering pronephros, mesonephros, metanephros, and ureteric bud induction
• Robbins and Cotran Pathologic Basis of Disease, 10th. Chapter 20, congenital anomalies of the kidney and urinary tract
• Harrison's Principles of Internal Medicine, 21st. Chronic kidney disease and congenital anomalies sections