 |
|
|
|
The following explanations refer to sample questions from the "2007 USMLE Step 1: Content Description and Sample Test Materials", which is subject to the copyright policy of the National Board of Medical Examiners® (NBME). The explanations were submitted by WikiTestPrep users and are therefore subject to the GNU Free Documentation License.
|
|
1. C
As skeletal muscle activity increases, the oxygen demand increases. The vascular supply to the area responds by dilating the arterioles in order to bring in more blood and supply the increased oxygen demand.
Choice A is incorrect because hydrostatic pressure in the capillaries is increased as blood flow increases.
Choice B is incorrect because contracting skeletal muscle will produce metabolites and release them into the capillaries.
Choice D is incorrect because contracting muscle cells will take up oxygen from the capillaries.
Choice E is incorrect because the vascular supply to the muscle is increased as the arterioles vasodilate, therefore vascular resistance decreases.
(91% of users answered this question correctly) Edit Explanation 2. D
Ear pain worsened on manipulation, with inflammation of the canal (i.e. external ear) is most consistent with external otitis (D), colloquially known as “swimmer’s ear.”
Acute otitis media (A) is an infection of the middle ear that usually presents with earache unaffected by external manipulation, with a red, bulging tympanic membrane on exam. Patients are often febrile to 102 degrees F (38.9 degrees C) or more. Children under 7 are more susceptible to this condition due to their shorter, more horizontal pharyngotympanic (aka auditory or Eustachian) tubes.
Bullous myringitis (B) is an infection of the tympanic membrane (TM) itself, in which bubbles filled with blood form on the TM surface.
Chronic otitis media (C) is incorrect both because of symptoms inconsistent with otitis media (see above), and the 1-day time course of the complaint is more suggestive of an acute process.
Mastoiditis (E) typically manifests with pain, tenderness, and/or swelling of the mastoid process. Further, such a short history of pain is inconsistent with mastoiditis, as it is usually a sequela of otitis media. Accordingly, children (not adolescents) are most frequently affected.
(81% of users answered this question correctly) Edit Explanation 4. E
The key point here is the increase in conjugated bilirubin, demonstrating that the liver is functioning properly. Combined with hepatomegaly, jaundice, bilirubinuria, and the described abnormal stools, this conjugated hyperbilirubinemia suggests an obstruction of the biliary system (E).
A defect in cholesterol synthesis (A) could adversely affect bile acid production and result in acholic, loose, clay-colored stools. However, such a defect should not affect bilirubin excretion and thus would not explain the hyperbilirubinemia seen in this infant. Furthermore, these defects in cholesterol synthesis are much less common than obstructions of the biliary system (E).
A deficiency of glucuronosyltransferase (B) is suggestive of Gilbert’s syndrome (GS) or the much less common Crigler-Najar syndrome (CDS). However, in GS and CDS the hyperbilirubinemia is due to unconjugated, not conjugated, bilirubin. Unconjugated bilirubin is insoluble, and therefore would not appear in the urine.
Hemolysis (C), like the glucuronosyltransferase disorders, results in an unconjugated, not conjugated, hyperbilirubinemia. Again, unconjugated bilirubin would not be seen in the urine.
Inflammation of the terminal ileum (D) in an infant with loose, clay-colored stools suggests celiac disease. Though iron absorption (and many other substances) may be affected, celiac disease should not cause hyperbilirubinemia. Crohn’s disease would also affect the terminal ileum and may have symptoms of malabsorption, but it too should not cause hyperbilirubinemia. Further, onset of Crohn’s is usually not seen until 15 – 30 years of age.
(81% of users answered this question correctly) Edit Explanation 5. E
The interosseous membrane (E) is a fibrous sheet between the radius and ulna, forming the radio-ulnar syndesmosis. It divides the forearm into anterior and posterior compartments, and transmits force from the radius to the ulna.
The annular ligament (A) comes off the lateral side of the ulna and encircles the proximal radius. It prevents translation of the proximal radius during pronation, and has no effect on axial forces like the force described here.
The bicipital aponeurosis (B), a.k.a. lacertus fibrosus, extends medially from the biceps tendon to reinforce the cubital fossa. It does not transmit force from the radius to the ulna.
The flexor retinaculum (C), a.k.a. transverse carpal ligament, forms the roof of the carpal tunnel. It does not transmit force from the radius to the ulna.
The intermuscular septum (D) is one of two (lateral or medial) structures arising from the humerus, separating the anterior and posterior compartments of the upper arm. It does not transmit force from the radius to the ulna.
(79% of users answered this question correctly) Edit Explanation 6. A
Nucleotide deletion in the codon for amino acid 11 (A) would cause a frameshift mutation. Such a mutation could result in a different amino acid at positions 11 and up, with the potential for an aberrant stop codon and early termination, as observed.
Nucleotide deletion in codon 20 (B) would cause a frameshift mutation. This mutation may result in a different amino acid 20, perhaps with termination thereafter. However, this option does not explain the difference observed in amino acids 11–19.
Nucleotide deletion in the intervening sequence (a.k.a. intron) (C) would not affect peptide sequence because introns are spliced out of the primary transcript, and thus are not translated.
Nucleotide substitution in codon 11 (D) may result in a different amino acid 11, but such a substitution would not change the rest of the nucleic acid sequence.
Nucleotide substitution in codon 20 (E) may result in a different amino acid 20, but such a substitution would not change the rest of the nucleic acid sequence.
7. B
Glucose is correct because erythrocytes do not have mitochondria. Accordingly, glucose is their only metabolic energy source, regardless of fasting status.
Beta-hydroxybutyrate can be used as a metabolic energy source by muscle or renal cortex tissue after an overnight fast, or by brain tissue in a prolonged fasting state. It is never an energy source for erythrocytes.
Free fatty acids can be metabolized by beta-oxidation in the mitochondria of cardiac and skeletal muscle. They are not an energy source for the brain or erythrocytes.
During fasting or exercise, pyruvate in the erythrocyte is converted into lactate then shuttled to the liver as part of the Cori Cycle. In other tissues, the mitochondrial enzyme pyruvate dehydrogenase can use pyruvate to generate acetyl-CoA for the citric acid cycle.
Triglycerides are the storage form of fatty acids, and are primarily found in the liver and adipose tissue.
8. B
The tremor described in the question stem is an intention tremor since it is most obvious as the patient's hand moves closer to his target (i.e. his coffee cup), and is not present while at rest. Intention tremors are common in essential tremor, intoxication, multiple sclerosis. They are a sign of cerebellar dysfunction. The cerebellum is split into the vermis and hemispheres. The cerebellar hemisphere (B) regulates the coordination of the ipsilateral extremities and therefore would be involved in an intention tremor. The cerebellar vermis (C) regulates the trunk and midline structures and its dysfunction would cause ataxia (balance problems).
Basal ganglia (A) involvement would cause rest tremor like those seen in Parkinson's.
Frontal eye field (D) would cause vision problems but not a tremor.
Motor nucleus of the thalamus (E) would most likely cause strength deficits, but not a tremor.
9. D
The bacteria in question is Clostridium difficile (the most common cause of antibiotic-associated diarrhea). C. difficile is a anaerobic gram positive rod that forms spores. These spores are only destroyed by autoclaving (exposure to saturated steam for 15 minutes, option D), because the spore shell is resistant to other forms of sterilization.
As a side note, using alcohol based cleaners (like Purell) will not sterilize your hands after you see a patient with C. difficile infection. Therefore, it is important to physically wash your hands with soap and water, in order to mechanically wash the spores off your hands.
(67% of users answered this question correctly) Edit Explanation 10. E
Due to the vomiting, this woman is losing HCl (the main stomach acid). Since she is unable to control the vomiting and is not eating she cannot adequately compensate for the acid loss. Therefore, she is in a state of metabolic alkalosis (pH>7.40) which eliminates options A, B, and C. Although her kidneys will eventually compensate by excreting HCO3, this process takes about a week to compensate, and therefore HCO3 will be increased (>24). In order to compensate, the respiratory drive decreases so that the body retains more CO2, and therefore pCO2 is increased (>40). Only option E has increased pH, pCO2 and HCO3.
Option A is metabolic acidosis
Option B is respiratory acidosis
Option C is Normal
Option D is respiratory alkalosis
(64% of users answered this question correctly) Edit Explanation 11. A
When the body is stressed, physiologically or pathologically, cortisol (option A) is produced in greater quantities than normal. In this case, severe burns is the stressor which would lead to an increase in cortisol. As a glucocorticoid, cortisol will increase protein catabolism which will increase nitrogen loss.
Erythropoietin (option B) is released by the kidneys to stimulate RBC production.
Insulin (option C) is released by the pancreas and regulates glucose levels and fat metabolism.
Parathyroid hormone (option D) is released by the parathyroid gland and regulates calcium, phosphate and Vit D.
Thyroxine (option E) released by the thyroid plays a part in increasing metabolic rate but would not specifically be involved in protein catabolism and thus nitrogen loss.
(86% of users answered this question correctly) Edit Explanation 12. C
Separation anxiety (option C) is a disorder of childhood that is most often seen in children 7-8 years old. The disorder manifests itself as an overwhelming fear of loss (usually of a parent) and can lead to physical complaints to avoid going to school or other activities. Separation anxiety is a normal part of development which occurs between 8-18 months; however, in a child >5 years old, separation anxiety that affects normal activity is described as a disorder.
Childhood schizophrenia (option A) develops before puberty and is presents as withdrawal, failure to develop a separate identity from the mother, and may include autistic like behaviors.
Latency is a Freudian stage of development that occurs from about age 6-puberty following the resolution of the Oedipal period. During this period the superego, the moral conscience is formed.
Socialized conduct disorder (option D) is is a persistent behavior that manifests as violation of the rights of others (stealing, fighting, malicious acts). In those older than 18 years, this disorder is classified as antisocial personality disorder. Socialized conduct disorder is related to ADHD and ODD (oppositional defiant disorder).
Symbiotic psychosis (option E) is classified as a pervasive developmental disorder of early childhood described by Margaret Mahler. Although separation anxiety is feature of the psychosis, it is accompanied by developmental and social retardation.
(89% of users answered this question correctly) Edit Explanation 13. E
This patient's serum bicarbonate is high, while her potassium is low. The potassium level in her urine is high. (Typical range is 50-150mEq per day). This indicates the use of a diuretic which blocks the reuptake of potassium.
Choice A is incorrect because aldosterone increases potassium secretion, therefore aldosterone deficiency would cause decreased potassium secretion.
Choice B is incorrect because hyperventilation causes respiratory alkalosis, resulting in decreased bicarbonate in the serum.
Choice C is incorrect because acidosis causes decreased potassium secretion. Also, diabetic ketoacidosis would cause additional symptoms and abnormally high blood glucose levels.
Choice D is incorrect because ingestion of anabolic steroids generally does not cause the type of potassium imbalance indicated by the lab values.
(68% of users answered this question correctly) Edit Explanation 14. D
The nucleus involved with circadian rhythms, sleep-wake cycle and melatonin regulation is the suprachiasmatic nucleus of the hypothalamus (option D).
Accessory optic nucleus (A) is involved involved in eye movements.
The lateral preoptic nucleus (B) is part of heat regulation in the hypothalamus.
The pretectal (C) is the area in the midbrain related to pupillary light reflex.
The supraoptic nucleus (E) of the hypothalamus regulates water balance and the produces ADH and oxytocin
(72% of users answered this question correctly) Edit Explanation 15. B
The cells expand in volume when they move from solution X into solution Y, indicating that water enters the cells. This occurs when cells are placed into a hypotonic solution: the level of solute in the cell is higher than in the surrounding solution, and the osmotic pressure created by the solute causes water to move into the cell.
Choice A is incorrect because placing cells into a more hypertonic solution would cause them to decrease in volume.
Choices C and D are incorrect because placing the cells into a isosmotic or isotonic solution would cause no change in cell volume.
(81% of users answered this question correctly) Edit Explanation 16. C
There is a one-one relation between the motor neuron and muscle cells (ie one motor neuron for each muscle cell). There is only one active end-plate from each motor neuron per muscle cell; therefore, option D is incorrect.
In addition, the action potential in the neuron will be transferred to create a distinct action potential in each muscle innervated by the neuron. The action potential is constant in its duration and in its amplitude; therefore options A and B are incorrect because these parameters cannot be changed.
Although the rate of synthesis of acetylcholine would increase the amount released by the axon terminal, the normal amount of acetylcholine released into the neuromuscular junction is enough to excite the muscle cell beyond the action threshold potential. Therefore, increasing the rate of synthesis (option E) would not effect the force of contraction.
Increasing the frequency of firing of individual motorneurons (option C) will increase the force of contraction because with each action potential firing in the muscle cell, more calcium enters the cell. Calcium drives the force of contraction and therefore with more calcium in the cell, the stronger the force of contraction will be.
(69% of users answered this question correctly) Edit Explanation 17. E
In women (especially in the elderly or those with multiple vaginal deliveries), there is tendency for the urogenital diaphragm (E) to weaken causing incontinence due to increase in the abdominal pressure (coughing, sneezing, laughing). Strengthening these muscles (sphincter urethrae muscle (external urethral muscle), deep transverse perineal muscle, superficial transverse perineal muscle and perineal membrane) can often help this incontinence.
The detrusor (A) is the muscle around the bladder that helps expel urine by squeezing the bladder.
The obturator internus (B) and the piriformis (C) stabilize the hip.
The rectus abdominis (D) is muscle of the abdominal wall.
19. B
The increased pulse and decreased blood pressure indicate a loss of fluid (volume contraction.) The patient's hemoglobin, urea nitrogen, and sodium all rise to above the normal values by Day 3, whereas his glucose and creatinine remain normal. These changes are consistent with an hyperosmotic volume contraction, a loss of water without losing solute. This typically occurs due to dehydration.
Choice A is incorrect because renal failure would decrease creatinine clearance.
Choice C is incorrect because diabetic ketoacidosis would cause increased blood glucose levels.
Choice D is incorrect because gastrointestinal hemorrhage would present with additional symptoms and hemoglobin would decrease.
Choice E is incorrect because SIADH would cause decrease in urine volume and retention of fluid, not loss of fluid.
(84% of users answered this question correctly) Edit Explanation 20. A
In the question stem, the gram positive cocci in clusters are most likely Staph aureus and therefore, the answer choices must pertain to specific attributes of S. aureus. S. aureus expresses protein A which will bind to IgG and fibrinogen receptors (S aureus clumping factor) which will bind fibrinogen (option A)
IL-1 and factor VIII (B) are produced by the body and therefore will not identify S aureus.
Properdin and platelet factor 3 (C) are related to complement and platelet aggregation.
Prothrombin and C3b (D) are related to thrombin (coagulation cascade leading to clot formation) and complement factor part of immune response.
Transferrin is a iron carrier in the blood and plasminogen is precursor for plasmin which is an enzyme involved in fibrinolysis (clot busting).
21. B
The symptoms and histology are typical for diverticulitis, the inflammation of a false pocket (diverticulum) consisting of mucosa and submucosa which have herniated through the muscularis layer. Diverticulitis affects older persons and typically presents with abdominal pain and rectal bleeding. Complications frequently include bowel stenosis, peritonitis, and abscess formation.
Choice A, C and D are incorrect because colon tumors typically present as polyps rather than pockets.
Choice E is incorrect because volvulus, an abnormal twisting of the intestine which impedes blood flow, typically has a sudden onset and does not display the histology shown here.
(78% of users answered this question correctly) Edit Explanation 22. A
The description of the bacteria is significant because it identifies the organism as group B streptococcus. Group B strep is a very common causes of newborn meningitis and sepsis, and it is most often found colonizing the mother's vagina (option A).
The newborn's GI tract (B) is not completely sterile, but group B strep is not part of the flora.
Colonization of the newborn's nasopharynx (C) is incorrect because the infection is not transmitted by the respiratory route, nor is it transplacental infection (D) (For transplacenta think TORCH), nor would it colonize the umbilical cord remnant (E) (an infection here would most likely be from an anaerobic bacteria).
(92% of users answered this question correctly) Edit Explanation 23. C
The clinical picture suggests a diagnosis of Gaucher’s disease. Gaucher’s disease is the most common variant of a group of diseases called lysosomal storage diseases which are caused by deficiency in any one of many lysosomal enzymes. Gaucher’s disease occurs due to the deficiency of beta-glucocerebrosidase, a lysosomal hydrolase (choice C) that converts glucocerebroside to cerebroside. Glucocerebroside accumulates and can then cause the clinical picture of mental retardation and hepatosplenomegaly.
Hormone-sensitive lipase (choice A) is an enzyme found in the cytosol of adipocytes that hydrolyzes triglycerides to free fatty acids and glycerols.
Lipoprotein lipase (choice B) is an enzyme found in endothelial cells lining capillaries that hydrolyzes triglycerides circulating in chylomicrons and VLDLs into free fatty acids and glycerols.
Sphingolipid synthase (choice D) is an enzyme that uses ceramide as a substrate for sphingomyelin production.
Tissue phospholipase (choice E) is an enzyme that hydrolyzes phospholipids into free fatty acids and other lipophilic products.
(64% of users answered this question correctly) Edit Explanation 24. A
Measles is an enveloped, single-stranded, negative sense RNA virus with a helical nucleocapsid. The hemagglutinin (HA) (choice A) viral protein is an envelope protein that binds sialic acid on cells allowing the virus to be endocytosed. Antibodies targeting viral envelope proteins have the best chance of binding their targets and providing the best immunity.
Matrix (choice B), nonstructural (choice C), nucleocapsid (choice D), and polymerase (choice E) proteins are shielded from their respective antibodies by the viral envelop and continue to be hidden once the virus is endocytosed into the host cell. Antibodies to the aforementioned proteins will not provide as much immunity from disease as antibodies to envelop proteins.
25. B
This patient has heart failure, hypotension, and no signs of atherosclerosis in the setting of alcohol abuse, sedentary lifestyle, and poor dieting. The most likely diagnosis is dilated cardiomyopathy secondary to alcohol abuse. Dilated cardiomyopathy is the most common cardiomyopathy and etiologies include alcohol abuse, thiamine deficiency, coxsackie viral infection, cocaine abuse, doxorubicin toxicity, and idiopathic. Ultimately, patients suffer from systolic dysfunction with eventual cardiac arrest. The most appropriate recommendation for this patient is to remove the source of his cardiomyopathy—alcohol (choice B).
Diet (choice C), exercise (choice A & D), and weight loss (choice E) will definitely have cardio-protective effects; however, removing the etiology of the patient’s heart failure needs to have highest priority in the management of this patient.
(68% of users answered this question correctly) Edit Explanation 26. C
Recent studies have demonstrated that grapefruit juice contains chemicals called furanocoumarins which have been shown to inhibit drug metabolism by interfering with the hepatic cytochrome P450 system, especially CYP3A4. Inhibiting a drug’s hepatic metabolism can lead to higher peak serum concentrations (choice C) of that drug.
Hepatic CYP3A4 activity (choice A) would be decreased by the furanocoumarins in grapefruit juice.
Intestinal drug metabolism (choice B) should not be affected because grapefruit juice inhibits hepatic drug metabolism.
Total body clearance (choice D) and volume of distribution (choice E) of the aforementioned drugs should both decrease. Volume of distribution is the ratio of the amount of drug in the body versus the plasma drug concentration. If the plasma drug concentration increases because drug metabolism is decreased, then volume of distribution will decrease. Clearance is the ratio of the rate of elimination of a drug to the plasma drug concentration. If the plasma drug concentration increases, then clearance will decrease.
(63% of users answered this question correctly) Edit Explanation 27. B
Finasteride inhibits 5-alpha-reductase, interfering with the conversion of testosterone to dihydrotestosterone (DHT) (choice B). DHT is the primary mediator in the pathogenesis of benign prostatic hyperplasia, causing hyperplasia of glandular and stromal cells.
Androstenedione (choice A) is the precursor to both male and female sex hormones. It can be formed from either dehydroepiandrosterone converted by 3-beta-hydroxysteriod dehydrogenase or 17-alpha-hydroxyprogesterone converted by 17,20 lyase.
Estradiol (choice C) is the major estrogen in humans. Estradiol can be formed by the aromatization of testosterone. Additionally, androstenedione can be aromatized to estrone (choice D) and then converted to estradiol by 17-beta-hydroxysteriod reductase.
Testosterone (choice E) is formed by converting androstenedione via oxidoreductase.
(90% of users answered this question correctly) Edit Explanation 28. B
Steadily increasing beta-hCG concentration status post removal of hydatidiform mole is suggestive of malignant gestational trophoblastic disease (GTD). Choriocarcinoma (choice B) is a GTD that is commonly associated with increased beta-hCG concentrations and hydatidiform moles, making it the most likely diagnosis in this patient.
Adrenal adenoma (choice A) and pituitary insufficiency (choice D) are not commonly associated with either hydatidiform moles or increased concentrations of beta-hCG.
Ectopic pregnancy (choice C) or a second noninvasive mole (choice E) could be possible explanations for an increased beta-hCG concentration, but is much less likely than a malignant GTD in this clinical scenario.
(85% of users answered this question correctly) Edit Explanation 29. D
This patient is exhibiting signs and symptoms of orthostatic (postural) hypotension in the setting of decreased intravascular volume secondary to prolonged diarrhea. Orthostatic hypotension is diagnosed when, within 2-5 minutes of standing, one or more of the following are present: (1) 20mmHg or more fall in systolic pressure; (2) 10mmHg or more fall in diastolic pressure; (3) symptoms of cerebral hypoperfusion. There are many etiologies of orthostatic hypotension which include decreased intravascular volume, autonomic insufficiency, alcohol, medications, and aging. The patient’s prolonged diarrhea has left her in a volume depleted state, causing the apparent signs and symptoms. The best initial therapy for this patient is to replete her intravascular volume with intravenous fluids that are as similar to her plasma as possible. The patient has lost both water and electrolytes in her diarrhea, both of which need to be replaced. Of the answer choices 0.9% Saline (choice D) both resembles plasma the most and is cost-effective. 0.9% Saline is also known as normal saline and is comprised of 154mEq of sodium and 154mEq of chloride.
Desmopressin (choice A) is a synthetic arginine vasopressin analogue that has antidiuretic effects. It can be used to treat patients with central diabetes insipidus. This patient most likely has high levels of ADH in her volume deplete state and would benefit more with IV hydration.
5% Dextrose in water (choice B) is a type of IV fluid; however, it is not as physiologically similar to plasma as 0.9% Saline. 5% dextrose in water is essentially water with dextrose without any electrolytes. Furthermore, as the dextrose will be rapidly metabolized, the water supplied distributes across both the extracellular and intracellular fluid compartments. This leads to a reduced amount of intravascular volume repletion relative to supplying saline.
Fresh frozen plasma (choice C) is a blood product with strict indications, only given to patients to replace clotting factors, not to correct volume status.
Methoxamine (choice E) is an alpha-1-adrenergic receptor agonist that induces vasoconstriction. This patient would benefit more from correcting volume status with IV fluids than with a vasoconstrictor.
30. E
This patient has a productive cough with unilateral signs of alveolar consolidation as demonstrated by right-sided bronchial breath sounds and increased tactile fremitus. In this clinical setting, lobar pneumonia (choice E) is the most likely diagnosis.
Asthmatic bronchitis (choice A) is characterized by a nocturnal cough, episodic expiratory wheezing (inspiratory as well when severe), and decreased breath sounds due to air-trapping.
Bullous emphysema (choice B) is characterized by progressive dsypnea, diminished breath sounds, and decreased tactile fremitus due to air trapping. A productive cough is not commonly associated with this condition.
Chronic bronchitis (choice C) is defined as a productive cough for at least 3 months for 2 consecutive years.
Congestive heart failure (choice D) can lead to bilateral pleural effusions as the pulmonary capillary hydrostatic pressure exceeds the capillary oncotic pressure and fluid moves into the pleural space. Clinically, CHF can be characterized by bilateral decreased breath sounds, crackles at both lung bases, decreased tactile fremitus bilaterally. Fremitus is decreased or absent when the transmission of vibrations from the larynx to the surface of the chest is impaired. Etiologies include obstructed bronchus, COPD, separation of the pleural surfaces by fluid, fibrosis, infiltrating tumor, or a very thick chest wall.
(90% of users answered this question correctly) Edit Explanation |