USMLE step 1 :: Q 29-31 :Answers

Question Number 29

Correct Answer: D

Explanation:
The history is strongly suggestive of posterior urethral valves . The neonates who are not recognized at birth most commonly present within a few weeks with urosepsis, dehydration, and electrolyte abnormalities In the male neonate, posterior urethral valves are suspected when there is a palpably distended bladder and the urinary stream is weak. If the obstruction is severe and goes unrecognized during the neonatal period, infants may present later in life with failure to thrive due to uremia or sepsis caused by infection in the obstructed urinary tract. With lesser degrees of obstruction, children present later in life with difficulty in achieving diurnal urinary continence or with UTI. The diagnosis is established by voiding cystourethrography or perineal ultrasonography.

Question Number 30

Correct Answer: C

Explanation:
In sickle cell disease the hypoxemic and hypertonic environment of the renal medulla encourages sickling of red blood cells circulating through this region. This leads to the development of papillary infarction. A defect in urinary concentration resulting in a tendency toward volume depletion is one of the best-characterised abnormalities in sickle cell nephropathy. Obliteration of the vasa recta compromises the operation of the medullary countercurrent system and impairs the ability to generate and maintain medullary solute gradients. The concentrating defect may also be seen in the sickle trait. A defect in urinary acidification is common and manifested as distal renal tubular acidosis with hyperkalemia and hyperchloremic metabolic acidosis (type 4 renal tubular acidosis). The acidification defect is not usually observed in patients with sickle trait. Painless gross hematuria has been estimated to occur in up to 50% of patients with sickle cell nephropathy. It also occurs in patients with Hb SA or Hb SC. Eecurrent papillary infarction leads to papillary necrosis. Sickle cell crisis, dehydration, hypoxemia, and the use of non-steroidal anti-inflammatory drugs all predispose to papillary necrosis. Renal papillary necrosis is often silent, but it may progress to chronic renal insufficiency and predispose the patient to repeated urinary tract infections.

 

Question Number 31

Correct Answer: E

Explanation:
This study shows selection bias. Selecting a population of working population is selective since working people are healthy enough to work and therefore less likely to suffer from stroke. By definition a selection bias produces a subject pool that is not representative of the generalised population