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NBME 24 Answers

nbme24/Block 3/Question#20

A 33-year-old woman comes to the physician because ...

Defect in a cell membrane anchor protein

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 +8  upvote downvote
submitted by m-ice(124),

This woman has Paroxysmal Nocturnal Hemoglobinuria. This most often presents in a young adult who has episodes of dark urine in the middle of the night or when waking up in the morning. It's caused by complement activity directly against the patient's own RBCs. Certain glycolipids are needed on the RBC surface to prevent attack from complement, the most notable of which are CD55 and CD59. Patients with PNH have a somatic mutation in which they lost function of a PIGA enzyme needed for proper presentation and attachment of CD55/CD59 on the RBC surface. Therefore the answer is a defect in a cell membrane anchor protein. Without this, complement attacks RBCs.

usmleuser007  I knew the disorder and its pathophysiology. But sometimes the answer choices are so wordy or colorful that you still get it wrong.... +9  
sunshinesweetheart  I got this one right but now upon review I'm having trouble ruling out hereditary spherocytosis ("abnormal cell morphology") answer choice. It helps that the dark urine is in the mornings, but is it officially ruled out because of her age? like this is obvi an acquired mutation if someone's 33? +  
krewfoo99  @sunshinesweetheart Hereditary Spherocytosis is a autosomnal dominant condition. The patient in the question stem has had dark urine since the past 2 months (acute presentation). Since spherocytosis is hereditary, it wont be present as a acute condition +  

 +0  upvote downvote
submitted by sweetmed(38),

The Ham test is a test used in the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH). The test involves placing red blood cells in mild acid; a positive result (increased RBC fragility) indicates PNH