USMLE-STEP-3 Online Practice Questions and Answers

Correct Answer: D Section: (none)

Explanation:

The patient has hypotonic hyponatremia, which can lead to increased water shifting into the brain, resulting in cerebral edema. This patient has nothing in history or physical examination to suggest a stroke or the presence of sepsis as the etiology of his altered mental status. Central pontine myelinolysis is a potentially devastating neurologic complication that can result from the treatment of hyponatremia, not hyponatremia itself. While respiratory acidosis could potentially contribute to this patient's change in mental status, cerebral edema due to hypotonicity is the most likely etiology. The patient's laboratory studies indicate a low plasma osmolality with an inappropriately increased urine osmolality. With this degree of hypotonicity, the urine should be maximally dilute (osmolality of <100 mOsmol/kg H2O). The high urine osmolality suggests the presence of antidiuretic hormone. In psychogenic polydipsia, the urine would be maximally dilute. Choice C is unlikely since his physical examination does not suggest volume depletion; furthermore, the patient is taking a calcium channel blocker, not a diuretic, for the treatment of his hypertension. Decreased expression of renal collecting duct water channels would lead to water wasting and, thus, the development of diabetes insipidus and hypernatremia. The patient has symptomatic hypotonic hyponatremia with signs of cerebral edema.

This requires immediate attention. Choices A, C, and E are essentially hypotonic solutions which should be withheld in patients with hyponatremia. The serum sodium in this case should be increased by at least 5% for the treatment of cerebral edema. The use of 0.9% saline would require nearly 5 L of infusate to address this cerebral edema. This could lead to pulmonary edema and volume overload. The use of hypertonic saline (3% saline) is the ideal solution to use in this scenario, as the infusion of 3% saline will correct the symptoms while avoiding volume overload. As in all cases of hyponatremia management, frequent serum sodium assays are necessary in order to avoid too rapid of a correction, which could result in neurologic injury--pontine myelinolysis.

Correct Answer: C Section: (none)

Explanation:

V. vulnificus is associated with sepsis in patients with liver disease who eat raw oysters or those with salt water contamination of wounds, like those caused by fish hooks. P. multocida is a cause of cellulitis caused by exposure to cat saliva as a result of a bite or a clawing injury. E. corrodens is associated with cellulitis caused by a human bite. Staphylococcus and Streptococcus are the most common causes of cellulitis. The Gram stain shows gram-negative, commashaped organisms typical for vibrios. Close attention should be paid to the wound site in the setting of a V. vulnificus infection. The wound site must be thoroughly cleaned and any necrotic tissue debrided. If necessary, fasciotomy or limb amputation should be performed. Antibiotic therapy should begin immediately as well. Use of a combination of doxycycline and a thirdgeneration cephalosporin such as ceftazidime is considered first-line. Quinolones may be considered as alternative therapy in the case of drug allergy or contraindication. A combination regimen using TMP-SMZ and an aminoglycoside is indicated for treatment in children since doxycycline and quinolones are contraindicated.

Correct Answer: C Section: (none)

Explanation:

This patient's presentation is most consistent with pulmonary edema from decompensated CHF. The BNP test has been found to be both sensitive and specific for the diagnosis of CHF. It can be a very useful test to order when a patient is dyspneic to help to determine if CHF is the cause. Troponin, CK-MB, and LDH are markers of damage to cardiac muscle and can be diagnostic in a MI. While MI can be a cause of CHF, and most patients presenting with CHF will have cardiac enzymes drawn as part of their evaluation, cardiac enzymes are neither sensitive nor specific for CHF. Similarly, a CXR can determine the presence of pulmonary edema but not its cause.

Acute pulmonary edema secondary to CHF will require management with diuresis for acute symptomatic relief. ACE inhibitors and beta-blockers do decrease mortality and morbidity in CHF; however their use in acute decompensated heart failure is suspected as they may induce hypotension and further cardiogenic shock. Digoxin is used for symptomatic relief either when other modalities fail or when rate control from atrial fibrillation is an issue. In patients with CHF and atrial fibrillation, beta-blockers have shown better effect and reduced morbidity than digoxin. Nevertheless, in the acute setting of decompensated heart failure with pulmonary edema, diuresis is the optimal initial treatment, not digoxin. In chronic heart failure, digoxin is reserved for patients with systolic failure that are symptomatic despite adequate ACE inhibitor and beta-blocker use. Furosemide is effective in treating the acute pulmonary edema associated with CHF by virtue of its potent diuretic action, which rapidly eliminates excess body fluid volume.

Correct Answer: A Section: (none)

Explanation:

FHH, or familial benign hypercalcemia, is a rare condition characterized by asymptomatic or mildly symptomatic hypercalcemia. It is inherited as an autosomal dominant trait and the parathyroid glands are usually normal in size. The basis for the development of FHH appears to be mutations in the calcium-sensing receptor gene which regulates the parathyroid gland set point and modulates the extracellular calcium concentration. The condition may be mistaken for primary hyperparathyroidism because, in both conditions, the serum calcium and parathyroid hormone levels are elevated with a concomitant low serum phosphate. The distinction is made by obtaining a 24-hour urine calcium excretion level. In patients with FHH, the urine calcium level is low, whereas in primary hyperparathyroidism the level is high. The distinction is important, as patients with primary hyperparathyroidism benefit from surgery and those with FHH do not.

Correct Answer: A Section: (none)

Explanation:

Explanations:

Understanding the concepts of sensitivity, specificity, PPV, and NPV is crucial to interpreting diagnostic test

results. Sensitivity is defined as the percentage of people who have a disease who test positive for that

disease. Specificity is defined as the percentage of people who are free of a disease who test negative.

These two concepts are considered characteristics of the specific test in question and are independent of

the prevalence of the disease in the population. The PPVs and NPVs of a test are the test's clinical

characteristics and these concepts are directly related to the prevalence of the disease in the population.

The PPV is the percentage of people who have a positive test result who actually have the disease.

Similarly, the NPV is the percentage of people who have a negative test result who don't have the disease.

While the definitions may seem subtly different, the implications are significant. The usual way to calculate

sensitivity, specificity, PPV, and NPV is with the table, using the following definitions

The definitions of sensitivity, specificity, PPV, and NPV would then be: Sensitivity = a/(a + c)

Specificity = d/(d + b)

PPV = a/(a + b)

NPV = d/(d + c)

The specific information in this question comes from an article describing a study of B-type natriuretic

peptide (BNP) for the diagnosis of CHF. The data presented show that 106 persons tested positive using

the BNP test. After comparison with the "gold standard," in this case, a review by expert cardiologists, 95 of

these 106 were determined to truly have CHF. Therefore, 11 of the 106 were false positives and 95 were

true positives. Similarly, 144 persons tested negative using the BNP test. Of these, 142 were confirmed as

true negatives and 2 were determined to be false negatives.

Putting these numbers into a table reveals:

The calculations then become:

Sensitivity = a/(a + c) = 95/(95 + 2) = 98%

Specificity = d/(d + b) = 142/(142 + 11) = 93%

PPV = a/(a + b) = 95/(95 + 11) = 90%

NPV = d/(d + c) = 142/(142 + 2) = 99%

Correct Answer: A Section: (none)

Explanation:

Amyloidosis is caused by the deposition of an abnormal proteinaceous material between cells. The majority of the cases are idiopathic, but a small percentage is secondary to chronic infection or inflammation, plasma cell dyscrasias, or immune diseases. One of the characteristic presentations of amyloidosis is splenic infiltration and splenomegaly caused by deposition of amyloid in the follicular regions. Grossly, the spleen has a diffuse, pink, glassy, waxy appearance with obliteration of the white pulp. Amyloid infiltration can also affect the kidneys, liver, and heart. Clinical symptoms are usually due to functional impairment of the diseased organ. The diagnosis of amyloidosis is made by tissue biopsy or, more recently, by fat-pad biopsy looking for amyloid deposits. With Congo red stain, amyloid appears red; with polarization, it shows an apple-green birefringence, which is diagnostic of amyloid.

Correct Answer: B Section: (none)

Explanation:

Chronic lymphocytic leukemia is a disease that presents generally over the age of 50 with a male predominance. For a long time many of these patients remain asymptomatic and, when they do present, the symptomatology is nonspecific, with generalized lymphadenopathy and hepatosplenomegaly. The peripheral lymphocyte count is generally high and composed of small lymphocytes. A low percentage of patients develop autoantibodies directed against red cells or platelets, which produces autoimmune hemolytic anemia or thrombocytopenia. Although the disease progresses and relapses in spite of the chemotherapy treatment, the overall median survival is 46 years, but this appears to be very variable. Some patients may survive longer than 10 years. All of the parameters for a worse prognosis have to be measured before a final statement of prognosis can be made. The lymph node architecture is diffusely effaced by a population of small lymphocytes, which contain nondiscernible cytoplasm and inconspicuous nucleoli. Mitotic activity is rare, focal proliferation centers with an increase in the number of mitotic activity cells are seen.

Correct Answer: D Section: (none)

Explanation:

Chronic lymphocytic leukemia is a disease that presents generally over the age of 50 with a male predominance. For a long time many of these patients remain asymptomatic and, when they do present, the symptomatology is nonspecific, with generalized lymphadenopathy and hepatosplenomegaly. The peripheral lymphocyte count is generally high and composed of small lymphocytes. A low percentage of patients develop autoantibodies directed against red cells or platelets, which produces autoimmune hemolytic anemia or thrombocytopenia. Although the disease progresses and relapses in spite of the chemotherapy treatment, the overall median survival is 46 years, but this appears to be very variable. Some patients may survive longer than 10 years. All of the parameters for a worse prognosis have to be measured before a final statement of prognosis can be made. The lymph node architecture is diffusely effaced by a population of small lymphocytes, which contain nondiscernible cytoplasm and inconspicuous nucleoli. Mitotic activity is rare, focal proliferation centers with an increase in the number of mitotic activity cells are seen.

Correct Answer: D Section: (none)

Explanation:

Hypothyroidism results from inadequate thyroid hormone production or a defect in thyroid hormone receptor activity. Hypothyroidism can be congenital or acquired. Most infants with congenital hypothyroidism are asymptomatic at birth. Feeding difficulties, choking spells, and somnolence often present during the first month of life. Respiratory distress can also occur in part due to the large tongue and nasal obstruction. On physical examination, you may find a large abdomen, umbilical hernias, subnormal temperature, cold skin, murmurs, or bradycardia. Iodine is absorbed in the GI tract as iodide. Iodide is concentrated in the thyroid gland and four atoms are incorporated into each molecule of thyroxine. Profound dietary deficiency of iodine will result in hypothyroidism and is the most common cause of goiter in the world. Rickets results from a deficiency of vitamin D.

This condition predominately affects the long bones and skull. Vitamin C deficiency results in scurvy, a condition with impaired collagen formation. The clinical manifestations may include changes in the gums, loosening of teeth, brittle bones, and swollen joints. Pallor is the most important sign of iron-deficiency anemia. Children may also have the desire to ingest unusual substances such as ice or dirt. Finally, hyponatremia and hypoglycemia are the prominent presenting signs of adrenal insufficiency in infants

Correct Answer: C Section: (none)

Explanation: Multiple myeloma is a plasma cell dyscrasia that is characterized by involvement of the skeleton in multiple sites. The characteristic x-ray shows punched-out bone lesions that are very easily seen in the calvarium. Extension of the disease to lymph nodes and extranodal sites, such as skin, can be seen. The bone marrow biopsy and smears reveal an increased number of plasma cells, which usually constitute greater than 20% of all of the cells. The cells either diffusely infiltrate and replace the marrow elements or can be seen scattered throughout the hematopoietic elements. The neoplastic plasma cells have a perinuclear hof and an eccentrically placed nucleus which allows the recognition. In 99% of patients with multiple myeloma, electrophoretic analysis reveals increased levels of IgG in the blood, light chains (Bence-Jones proteins) in the urine, or both. The monoclonal IgG produces a high spike when seen in the serum or in the urine, subject to electrophoresis. In general, the quantitative analysis of the monoclonal IgG is more than 3 g. The clinicopathologic diagnosis of multiple myeloma rests on radiographic and laboratory findings. Marrow examination may reveal increased plasma cells or sheet-like aggregates that may completely replace the normal elements. The prognosis for this condition is variable, but generally poor.