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The Anatomy of a Didactic Question


Question

Which of the following is a risk factor for struvite urinary tract stones?


Most Correct
Least Correct
  • D.
    Recent urinary tract infection.Struvite stones occur due to upper urinary tract infections caused by urease-producing organisms, such as Proteus or Klebsiella.
  • A.
    High urinary calcium is a risk factor for calcium-based renal and ureteral stones. Urinary calcium is increased by high oxalate diets, low water intake, low calcium dietary intake, and calcium supplementation.
  • B.
    History of gout attacks is a risk factor for uric acid-based stones.
  • C.
    Male sex is a risk factor for calcium-based stones. Struvite stones are more common in women since women are more likely to have urinary tract infections.

The Anatomy of an Explanation

Nephrolithiasis refers to the presence of crystalline stones (calculi) within the urinary system (kidneys and ureters). It is a relatively common medical problem. Most (80%) of stones within the urinary system are made up of calcium. Urinary stones can also be composed of uric acid, struvite (magnesium ammonium phosphate), and cystine. Prior personal history of nephrolithiasis is the most important risk factor. The risk for nephrolithiasis is based on urine composition, which can be affected by other medical conditions and lifestyle choices. High urine calcium, high urine oxalate, and low urine citrate each increase the risk of calcium stone formation. Dietary risk factors for calcium-based stones include low fluid intake, low calcium intake, high oxalate intake, and high animal protein intake. Patients who have a family history of nephrolithiasis are at increased risk. Medications that can crystallize in the urine include indinavir, acyclovir, sulfadiazine, and triamterene. Roux-en-Y gastric bypass surgery increases the amount of calcium oxalate excretion, which increases the rate of calcium oxalate stone formation. Risk factors for struvite stones include female sex and a history of urinary tract infections. Struvite stones occur due to upper urinary tract infections caused by urease-producing organisms, such as Proteus or Klebsiella. Patients with struvite stones have persistently alkaline urine (pH > 7.0). The classic symptoms and signs of nephrolithiasis include unilateral flank pain and hematuria (gross or microscopic). The pain varies from mild and barely noticeable pain to excruciating pain. The pain is often described as waxing and waning with waves of paroxysms. The site of obstruction determines the location of pain. For example, upper ureteral obstruction can lead to flank pain or tenderness, whereas lower ureteral obstruction can cause pain to radiate into the ipsilateral testicle or labium. Patients may also be diagnosed with incidental nephrolithiasis when abdominal imaging is performed for other reasons. The diagnosis of nephrolithiasis should be confirmed by the presence of a stone on imaging. The preferred imaging modality is computed tomography (CT) of the abdomen and pelvis without contrast. Computed tomography (CT) scan of the abdomen and pelvis can confirm the presence of a stone, determine the location of the stone, and suggest the mineral composition of a stone. The preferred imaging modality in pregnant patients is an ultrasound of the kidneys and bladder. Ultrasound of the kidneys and bladder reliably detects hydronephrosis and does not involve ionizing radiation. Most patients with acute renal colic due to nephrolithiasis are managed conservatively with medical therapy including pain medication and hydration until the stone passes. Nonsteroidal anti-inflammatory drugs (NSAIDs) appear to be at least as effective as opiates. Patients can typically be managed at home if they can tolerate oral medications and oral fluids. Hospitalization is required for patients who cannot tolerate oral intake or who have uncontrollable pain. Medications, such as tamsulosin and nifedipine, can also be used to facilitate stone passage. These medications are particularly helpful for patients who have stones between 5 mm and 10 mm in size. Patients who are passing a renal stone should be instructed to strain their urine for several days and bring in any stone that passes for stone analysis.

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