A 21-year-old man is brought to the emergency department due to diffuse muscle aches and weakness. He has also noticed darkening of his urine. The patient recently joined the military and was participating in rigorous training exercises in hot weather earlier in the day. He has no significant medical conditions and takes no medications. Medical evaluation and laboratory testing performed prior to military enlistment showed no abnormalities. Temperature is 36.7 C (98 F), blood pressure is 100/60 mm Hg, pulse is 105/min, and respirations are 16/min. Physical examination shows dry mucous membranes and muscle tenderness over the bilateral thighs and calves. Laboratory results are as follows: Which of the following urine microscopy findings is most likely to be seen in this patient?

Question

A 21-year-old man is brought to the emergency department due to diffuse muscle aches and weakness.  He has also noticed darkening of his urine.  The patient recently joined the military and was participating in rigorous training exercises in hot weather earlier in the day.  He has no significant medical conditions and takes no medications.  Medical evaluation and laboratory testing performed prior to military enlistment showed no abnormalities.  Temperature is 36.7 C (98 F), blood pressure is 100/60 mm Hg, pulse is 105/min, and respirations are 16/min.  Physical examination shows dry mucous membranes and muscle tenderness over the bilateral thighs and calves.  Laboratory results are as follows:  Which of the following urine microscopy findings is most likely to be seen in this patient?

Quizplus · Accepted Answer

11ec160b_3a4f_ea9c_9249_99d80ff806fb_MD0001_00 This patient's presentation is consistent with rhabdomyolysis, which is characterized by skeletal muscle necrosis and the release of intracellular breakdown products into the circulation. The condition is most commonly caused by trauma, sepsis, drugs/toxins (eg, statins, alcohol, cocaine), and overexertion (particularly in hot climates). Patients classically have myalgia and weakness (predominantly in the proximal muscles, lower back, and calves) and dark urine (due to myoglobinuria). Laboratory studies often show markedly elevated creatine kinase levels and acute kidney injury with electrolyte disturbances (eg, hyperkalemia, hyperphosphatemia, hypocalcemia, metabolic acidosis). Acute kidney injury in rhabdomyolysis occurs due to myoglobin degradation and heme pigment release. Heme pigment causes acute tubular necrosis (ATN) through direct cytotoxicity and renal vasoconstriction (ie, ischemia). In ATN, injured tubular epithelial cells slough off into the tubular lumen, forming granular, muddy brown casts. Heme pigment in myoglobin cross-reacts with the urine dipstick reagent that detects hemoglobin, leading to a false-positive result for blood in urine; however, microscopy shows no red blood cells (RBCs). (Choices A, D, and F) Dysmorphic RBCs and RBC casts are typically seen in patients with glomerulonephritis. Dysmorphic RBCs have abnormal shapes due to deformation as they pass through the glomerular basement membrane and osmotic stress in the renal tubules. Normal-appearing (isomorphic) RBCs are seen in nonglomerular sources of hematuria, such as nephrolithiasis or urinary tract malignancies. (Choice B) Urinary eosinophils are suggestive of acute interstitial nephritis (AIN), although they may be associated with other conditions (eg, kidney transplant rejection, pyelonephritis). AIN results from immune-mediated tubulointerstitial injury often caused by medications (eg, nonsteroidal anti-inflammatory drugs, penicillins). Patients classically have some combination of rash, fever, and eosinophilia. (Choice E) Polymorphonuclear leukocytes in the urine indicate inflammation, which most commonly occurs due to infection. Interstitial nephritis, renal tuberculosis, and gonorrhea/chlamydia urethritis should be considered in patients with negative urine cultures (sterile pyuria). Educational objective: Rhabdomyolysis usually presents with myalgia, proximal muscle weakness, and dark urine (myoglobinuria) in the setting of trauma, sepsis, or overexertion. Kidney injury occurs due to heme pigment-mediated tubular injury, leading to acute tubular necrosis. Urine microscopy typically reveals granular, muddy brown casts.

A 21-Year-Old Man Is Brought to the Emergency Department Due