A 24-year-old man is brought in after being trapped under collapsed debris for six hours. He has swollen painful thighs, serum creatinine 4.8 mg/dL, potassium 6.2 mEq/L, and calcium 7.0 mg/dL. Urine dipstick tests strongly positive for blood, but microscopy shows no red cells and numerous muddy brown granular casts. The dipstick positivity is explained by which pigment?
- A Haemoglobin from intravascular haemolysis
- B Myoglobin released from injured muscle ✓
- C Free bilirubin conjugates in urine
- D Lysed red cells from haemorrhagic cystitis
Correct answer: B. Myoglobin released from injured muscle
Explanation
The dipstick orthotolidine reaction detects the haem moiety and cannot distinguish haemoglobin from myoglobin. Positive dipstick with absent red cells on microscopy indicates myoglobinuria from crush injury, causing pigment nephropathy and acute tubular necrosis, evidenced by muddy brown casts. Haemoglobinuria would require documented intravascular haemolysis with a low serum haptoglobin, and the absence of red cells excludes bleeding anywhere in the urinary tract.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.