Morning Quiz ☕️ 108

Further laboratory evaluation is most likely to reveal which of the following abnormalities in this patient?

Dr. Shoaib Ahmad Baig
March 01, 2025

Morning Quiz ☕️

A 13-year-old boy is brought to the emergency department with the following presentation:

Sudden onset of left-sided abdominal pain.
Blood in his urine
The pain waxes and wanes in intensity and does not improve with rest or position changes.
He has a lengthy history of similar pain episodes, but this is the first time he has experienced gross blood in urine.

Physical Examination:

Costovertebral angle tenderness on the left side.

Laboratory Findings:

Microscopic examination of the urine
Question:
Further laboratory evaluation is most likely to reveal which of the following abnormalities in this patient?

Options:
A. Aminoaciduria
B. Hypercalciuria
C. Hyperoxaluria
D. Hyperuricosuria
E. Hypocitraturia

Correct Answer 🎯: A. Aminoaciduria

Cystinuria is a specific type of aminoaciduria where cystine, ornithine, lysine, and arginine (COLA) are excreted in excess due to defective renal tubular reabsorption.
This is a hallmark of cystinuria and explains the hexagonal crystals seen in urine. 🧠 (Key insight: Hexagonal crystals = cystinuria)

⚙️

Genetic Mutation (Autosomal Recessive)
⬇
Defect in SLC3A1 or SLC7A9 genes (encoding cystine transporters)
⬇
⚙ Impaired renal reabsorption of cystine in the proximal tubule
⬇
↑ Cystine in urine (Cystinuria)
Cystine is poorly soluble in urine, especially at acidic pH
⬇
⚙ Supersaturation of cystine in the renal tubules

⬇
Formation of hexagonal crystals (seen on urine microscopy)

🔍 (Hexagonal crystals are like snowflakes, unique to cystinuria)
⬇
Cystine crystals aggregate to form cystine stones
⬇
Stones obstruct the urinary tract (left ureter in this case)
⬇
⚙ Obstruction → ↑ Pressure in renal pelvis
⬇
Stretching of renal capsule → Left-sided abdominal pain (waxing and waning, colicky)

🔍 (Think of a water balloon stretching and causing pain)
⬇
Stone movement causes trauma to the urothelium
⬇
Gross hematuria (first occurrence in this patient)

🔍 (Like a sharp rock scraping the lining of a pipe)
⬇
Obstruction and stretching of the renal capsule
⬇
⚙ Inflammation and irritation of the renal parenchyma
⬇
Left costovertebral angle tenderness on physical exam

🔍 (Like pressing on a bruised area)

Differential Table ⚖️

Differential ⚖️	Mechanism ⚙️	Why Incorrect? 🔴
Hypercalciuria	↑ Calcium in urine → Calcium oxalate/phosphate stones (not hexagonal crystals)	Cystine stones are not calcium-based; no evidence of calcium-related pathology.
Hyperoxaluria	↑ Oxalate in urine → Calcium oxalate stones (rhomboid crystals, not hexagonal)	No history of dietary oxalate excess or malabsorption; crystals are hexagonal.
Hyperuricosuria	↑ Uric acid in urine → Uric acid stones (rhomboid crystals, form in acidic urine)	No history of gout or high purine intake; crystals are hexagonal, not rhomboid.
Hypocitraturia	↓ Citrate in urine → Calcium phosphate stones (citrate inhibits stone formation)	Citrate deficiency is not specific to cystine stones; crystals are hexagonal.

Practice Questions ⏳️

Most Importantly Solve UWorld Questions with IDs:

814, 815

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Dr. Shoaib Ahmad