Correct Answer: D. Putty kidney
Putty kidney (also called "granular kidney" or "tuberculous kidney") is the end-stage renal tuberculosis characterized by extensive caseous necrosis, fibrosis, and calcification of the renal parenchyma, resulting in a non-functioning kidney with a putty-like consistency. The clinical presentation of sterile pyuria (pus in urine without bacterial growth on standard culture) is pathognomonic for renal TB, as the causative organism Mycobacterium tuberculosis requires special culture media (Löwenstein-Jensen medium) and is not detected on routine bacterial cultures. On imaging (plain radiograph or CT), putty kidney shows a shrunken, calcified kidney with loss of normal architecture—the entire organ appears as a calcified mass. The sterile pyuria combined with abdominal/flank pain in the context of TB-endemic India (high prevalence of extrapulmonary TB) and the radiological finding of a calcified, non-functioning kidney makes putty kidney the diagnosis. This represents the final stage of tuberculous pyelonephritis and typically requires nephrectomy as the kidney is non-functional and a potential source of infection.
Why the other options are wrong
A. Staghorn calculus — Staghorn calculus is a branching stone that fills the renal pelvis and calyces, typically associated with recurrent UTIs caused by urease-producing bacteria (Proteus, Klebsiella). It presents with bacterial pyuria and positive urine culture, not sterile pyuria. The imaging shows a branching radiopaque stone, not diffuse renal calcification. Staghorn calculi are more common in males with obstructive uropathy, not TB. B. Nephrocalcinosis — Nephrocalcinosis refers to calcium deposition within the renal parenchyma (medullary or cortical) due to hypercalcemia, hyperparathyroidism, renal tubular acidosis, or chronic pyelonephritis. It typically shows bilateral, symmetric calcification in the medullary pyramids. Unlike putty kidney, nephrocalcinosis does not present with sterile pyuria and does not show the characteristic shrunken, non-functioning kidney with complete loss of renal architecture seen in TB. C. La d Psoas calcification — Psoas calcification (or psoas abscess calcification) is calcification of the psoas muscle, typically secondary to spinal TB (Pott's disease). While it can occur in TB patients, it is a separate entity from renal pathology and would not directly cause sterile pyuria. The clinical presentation and imaging findings described (calcified kidney with loss of function) are characteristic of renal TB, not psoas involvement.
High-Yield Facts
- Sterile pyuria (pus without bacterial growth on routine culture) is pathognomonic for renal tuberculosis and requires special culture media (Löwenstein-Jensen) for M. tuberculosis isolation.
- Putty kidney is end-stage renal TB with complete caseous necrosis, fibrosis, and calcification resulting in a non-functioning, shrunken kidney requiring nephrectomy.
- Imaging findings in putty kidney: diffuse renal calcification, loss of normal renal architecture, shrunken kidney on CT/plain radiograph (not branching stone pattern).
- Incidence of renal TB in India: 10–15% of all TB cases have genitourinary involvement; renal TB is the most common site of extrapulmonary TB.
- Management of putty kidney: nephrectomy is indicated as the kidney is non-functional and a persistent source of infection and potential malignancy risk.
Mnemonics
PUTTY KIDNEY = TB End-Stage Pus without bacteria (sterile pyuria) → Urinary TB → Tuberculous → Terminology (putty = non-functioning) → Y (yes, needs nephrectomy). Sterile pyuria is the red flag that screams TB, not bacterial infection. Renal TB Stages (CUFF) Caseous (early parenchymal TB) → Ulceration (into collecting system) → Fibrosis (scarring) → Final (putty kidney, non-functioning). Each stage worsens; putty is the end.
NBE Trap
NBE pairs "sterile pyuria" with common stone disease (staghorn calculus) to trap students who reflexively link pyuria to bacterial UTI. The discriminator is the word "sterile"—this single word shifts the diagnosis entirely from stone/bacterial infection to TB.
Clinical Pearl
In India, any patient presenting with flank pain, sterile pyuria, and constitutional symptoms (fever, weight loss) should raise suspicion for renal TB until proven otherwise. Putty kidney is often discovered late because sterile pyuria is frequently missed on routine urinalysis, and the diagnosis requires high clinical suspicion and special mycobacterial culture techniques.
_Reference: Bailey & Love Ch. 76 (Urinary Tuberculosis); Harrison Ch. 125 (Tuberculosis); KD Tripathi Ch. 47 (Antituberculous drugs and TB pathology)_