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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.
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61. E
Inserting a single nucleotide between codons 135 and 136 shifts the reading frame, so that the codons which follow now read: UUU (new codon 136), AUG (new codon 137), UGU (new codon 138), UAA (new codon 139), with a leftover uridine at the end. However, the new sequence includes a stop codon, UAA (new codon 139), which means that the total number of amino acids will be 138 (choice E).
(66% of users answered this question correctly) Edit Explanation 62. E
Primary spontaneous pneumothorax presents with acute chest pain and shortness of breath. It is believed to result from rupture of a subpleural bleb (choice E), which is usually located in the apex of the lung. Blebs can be found in more than 75% of patients undergoing thoracoscopy for treatment of primary spontaneous pneumothorax. Also, patients with a spontaneous pneumothorax tend to be thinner and taller on average as is the case with this patient. An important distinction of the correct answer from the other answer choices is that it is the only choice that most likely will have an acute presentation in adulthood.
Bronchiectasis (choice A) and lung abscess (choice B) are both caused by infectious processes that would be unlikely in a previously healthy patient.
Panacinar emphysema (choice C) and pulmonary sequestration (choice D) would both most likely present much earlier in life. Panacinar emphysema is most commonly a result of alpha-1-protease inhibitor deficiency and pulmonary sequestration is a congenital malformation of the lower respiratory tract which results in a nonfunctioning mass of lung tissue.
(86% of users answered this question correctly) Edit Explanation 63. C
Group B Streptococci, or more specifically S. agalactiae, occasionally colonize the female reproductive tract and are common causes of neonatal meningitis. The best characterized virulence factors of Group B Streptococci are the capsular polysaccharides, which confer serotype specificity. The capsule provides virulence by inhibiting the deposition of complement components on the surface of the organism and therefore inhibiting ingestion (choice C) by cells of the immune system.
The other answer choices are examples of virulence factors of other organisms, but not Group B Streptococci.
(63% of users answered this question correctly) Edit Explanation 64. E
Given this patient's presentation and the finding of curved bacterial rods, the patient is most likely infected with Helicobacter pylori. H. pylori is a major cause of peptic ulcer disease as well as gastritis, both which can cause epigastric pain with a positive fecal occult blood test. H. pylori tests positive for oxidase, catalase, and urease. Therefore, there would most likely be increased urease activity in the antrum (choice E) of this patient.
Achlorhydria (choice A) and antiparietal cell antibodies (choice B) both describe changes seen in pernicious anemia.
Infection with H. pylori would most likely not cause cholecystitis (choice C) or an immunodeficient state (choice D).
(88% of users answered this question correctly) Edit Explanation 65. G
Sulfonylureas increase the secretion of insulin from pancreatic beta cells (choice E) by binding to ATP-dependent potassium channels causing an inhibition of the outflux of potassium. This causes a depolarization of the cell membrane, which opens voltage-dependent calcium channels and stimulates secretion of insulin-containing granules. In contrast, sulfonylureas do not decrease the secretion of insulin from the pancreas (choice B).
Changing the amount of glucose produced by the liver (choices A and D) is also not the mechanism of action of sulfonylureas. Metformin is an example of a drug that can decrease the amount of glucose produced by the liver.
Sulfonylureas also do not decrease the speed of carbohydrate absorption from the intestines (choice C).
66. C
This patient has regressed (choice C) because she is dealing with her problem in a way a child would, and she has therefore set back her maturational clock. This defense mechanism is seen in very sick patients as well as in children under stress who may begin wetting the bed.
Denial (choice A) is the avoidance of awareness of some painful reality, such as the denial of a newly diagnosed cancer.
Displacement (choice B) is a process whereby avoided ideas and feelings are transferred to some neutral person or object. An example of this is a wife yelling at her husband because she is angry at her boss.
Repression (choice D) is an involuntary withholding of conscious awareness.
Sublimation (choice E) is the only mature defense mechanism listed in the answer choices, involving replacement of an inappropriate desire with a desire or action that is considered appropriate, differing from displacement where the desire is replaced or transferred, but to a person or object that is still inappropriate. An example of sublimation would be a person who utilizes they're frustration or anger as competitiveness in sports.
67. A
Granulocyte colony-stimulating factor (choice A) increases neutrophil, eosinophil and basophil counts: the patient’s white blood cell differential shows an increase in the percentage of neutrophils from 9% to 90% after the growth factor was given. The percentage of eosinophils did not change, but since the total number of cells increased, the absolute eosinophil amount also increased. The percentage of lymphocytes decreased.
The other growth factors listed would not increase production of neutrophils. Transforming growth factor-beta (choice B) is made by macrophages, endothelial cells, and T-cells and is involved in B-cell maturation.
IL-6 (choice C) is an acute phase reactant made by macrophages, and
IL-8 (choice D) is involved in neutrophil migration and chemotaxis.
Macrophage colony-stimulating factor (choice E) would increase the percentage of cells in the monocyte-macrophage line, but that information is not provided in the question.
68. B
Point Y shows an elevated pCO2, but the pH is slightly below normal, indicating that the patient has compensated and is thus experiencing a chronic acid/base imbalance. In chronic obstructive pulmonary disease (choice B), patients retain CO2 and thus are in a state of chronic respiratory acidosis; the renal compensation is to retain bicarbonate and secrete H+ to correct the pH.
Adaptation to high altitude (choice A) would lead to a higher than normal pH, as a person would try to breathe in more oxygen would hyperventilate, blowing off CO2 and become alkalotic.
In diarrhea (choice C), the patient experiences metabolic acidosis, but the respiratory compensation would lower the pCO2, not raise it.
Similary, in the case of ingestion of a strong acid (choice D), the patient would hyperventilate to blow off CO2 to compensate for the metabolic acidosis, lowering the pCO2, not raising it.
Severe prolonged vomiting (choice E) leads to loss of acidic gastric contents which would lead to a metabolic alkalosis – since the body never “over-compensated” an acidic pH in the face of metabolic alkalosis is unlikely even with respiratory compensation, and especially not in an acute setting.
(62% of users answered this question correctly) Edit Explanation 69. C
Antimicrobial resistance in Escherichia coli is primarily conferred through transfer of a plasmid (choice C). This is true in the case of strains of Excherichia coli that carry a resistance to ampicillin. Without the need to produce the beta-lactamases needed to be resistant to ampicillin, it will not preferentially transfer the plasmid and it may be lost in subsequent generations.
The other four answer choices discuss changes to the actual gene which are not ways in which E. coli develops resistance.
(65% of users answered this question correctly) Edit Explanation 70. A
Haemophilus decreyi (choice A) is a Gram-negative coccobacillus that is typically grown on chocolate agar. Like the other answer choices, it is a sexually transmitted disease, but it is characterized by painful lesions in the genitalia.
Herpes simplex virus (choice B), like other viruses, cannot be Gram stained. The test of choice for identifying a herpes virus is a Tzanck test.
Neisseria gonorrhoeae (choice C) is a Gram-negative diplococci.
Treponema pallidum (choice D) is the bacteria that causes syphilis and is a spirochete.
Trichomonas vaginalis (choice E) parasitic flagellated protozoan.
(67% of users answered this question correctly) Edit Explanation 71. E
Selective constriction of the efferent arterioles will cause the changes described in choice E. It will increase GFR by causing an increase in the glomerular capillary pressure causing an increase in the net pressure in the glomerulus. There will also be decreased renal blood flow as constriction of either the afferent or efferent arterioles will cause a decrease in perfusion to the kidneys. As the filtration fraction is the GFR devided by renal plasma flow, which is effectively renal blood flow, this number will increase as GFR has increased and renal blood flow has decreased.
The other choices give an incorrect combination of answers.
(82% of users answered this question correctly) Edit Explanation 72. D
ADH (vasopressin) is secreted by the posterior pituitary gland. Its secretion is in response to either activation of baroreceptors in the veins, atria and carotid bodies in response to reduced plasma volume, or by activation of osmoreceptors in the hypothalamus in response to increased plasma oncotic pressure. The patient in this question has orthostatic hypotension and poor skin turgor, clinical signs suggesting volume depletion. Likewise, plasma volume is low and ADH levels are responsively increased. Because plasma sodium concentration is lower than normal, the renin-angiotensin-aldosterone system will be activated and the concentrations of all three will be increased. ANP is secreted by atrial myocetes in response to increased blood pressure, the opposite of this patient. In dehydrated states, urine osmolality is higher than plasma/serum osmolality due to the action of ADH increasing permeability of the collecting duct to reabsorb water.
73. C
This patient is exhibiting signs of Wernicke's encephalopathy, a condition of thiamine deficiency. The classic triad of Wernicke's is encephalopathy, ophthalmoplegia in the form of lateral gaze nystagmus, and ataxia. Chronic alcoholics and indivduals undergoing TPN without B1 supplements are predisposed to B1 deficiency. Wernicke's may be precipitated by carbohydrate heavy meals because thiamine is a cofactor for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, key enzymes in glucose metabolism. Untreated Wernicke's encephalopathy may progress to Korsakoff syndrome, an irreversible condition characterized by psychosis, anterograde and retrograde amnesia, and confabulation.
(81% of users answered this question correctly) Edit Explanation 74. C
Cytomegalovirus (CMV) infection in immuncompetent hosts is usually subclinical, or at most presents as a mild form of mononucleosis similar to Epstein-Barr virus with mild fever, lymphadenopathy, splenomegaly and atypical circulating lymphocytes for several weeks. However, in immunocompromised hosts, such as the post-transplant patient above, CMV infection or reactivation of a latent CMV infection can manifest as a multisystem disease within 20-60 days. In addition to the aforementioned features of mononucleosis, CMV in immunocompromised hosts also manifests as pneumonia, hepatitis, gastritis and colitis. In late stage AIDS patients, CMV particularly causes a severe form of retinitis.
Adenovirus also causes a worsened form of its normal disease in immunocompromised hosts, but usually presents with severe gastroenteritis (vomiting and diarrhea) and pneumonia/bronchitis/ARD. Coxsackie virus usually presents in childhood as hand-foot-and-mouth disease, causing painful, vesicular, blisters in the oral mucosa, tonsillar pillars and palms and soles. Though influenza may cause a severely fatal pneumonia in immunocompromised hosts, it does not cause usually cause liver abnormalities. Parvovirus usually causes a chronic infection of the bone marrow and should be suspected in post-transplant patients with erythropoietin-resistant anemias.
(82% of users answered this question correctly) Edit Explanation 75. E
Between the ages of 7-9 months, an infant develops stranger anxiety for individuals who are not primary caregivers. Stranger anxiety is a normal developmental milestone. It manifests as crying and physical discomfort when not in the presence of a primary caregiver, and usually resolves by 24 months. It is commonly confused with the pathologic entity of separation anxiety disorder, a condition usually seen beyond the age of two years and classified as three or more of the following DSM-IV criteria:
- Recurring distress when separated from the subject of attachment (such as the mother or home)
- Persistent, excessive worrying about losing the subject of attachment
- Persistent, excessive worrying that some event will lead to separation from a major attachment
- Excessive fear about being alone without subject of attachment
- Persistent reluctance or refusal to go to sleep without being near a major attachment figure, like a mother
- Recurrent nightmares about separation
(76% of users answered this question correctly) Edit Explanation 76. B
When the USMLE question involves a traveler returning to a trip from a specific locale, it is usually a clue to the diagnosis. In this case, coccidiomycosis is a well-known fungal disease endemic to California, Arizona, New Mexico and Nevada, giving it the moniker "Valley fever." It usually presents several weeks after exposure as a flu-like illness with fever, myalgias and erythema nodosum over the shins. Patients who fail to clear the infection may progress to chronic pulmonary infection, pulmonary effusions or meningitis.
Blastomycosis and Histoplasmosis are also systemic fungal infections, but the travel history usually involves a recent visit to the Mississippi/Ohio River valley. M. pneuomniae causes a pneumonia syndrome, and M. marinum causes systemic skin nodules, but both are usually opportunistic infections of the immunocompromised.
(78% of users answered this question correctly) Edit Explanation 77. D
17 beta-estradiol is the major form of estrogen present in a woman's body from menarche to menopause. Its role in menstruation is primarily during the proliferative phase (day 1-14) to increase the size of the endometrium and prepare it for the progesterone-dominated secretory phase (day 15-28) during which an embryo will potentially implant and spiral vessels will penetrate the layers of the endometrium. Estradiol peaks on day 13, and a surge of positive feedback on luteinizing hormone (LH) by estradiol during this period causes the follicle to burst on day 14 of the cycle, releasing the egg. Progestone levels are at their lowest during this period, and it is only the secretion of progesterone by the remnants of the follicle, the corpus luteum, that causes a rise in progesterone levels in the second phase of the cycle.
(85% of users answered this question correctly) Edit Explanation 78. A
This patient is exhibiting signs of upper and lower motor neuron degeneration, suggesting a diagnosis of amyotrophic lateral sclerosis (ALS). Upper motor neuron degenerative signs include hyperreflexia, increased tone, and positive bilateral Babinski signs. Lower motor neuron degenerative signs include muscle atrophy, areflexia, fasciculations, and flaccid paralysis. Since ALS causes degeneration of both upper and motor neurons, this is the likely diagnosis for this patient.
Alzheimer dementia is not known to correlate with motor neuron loss. Guillan-Barre is usually post-viral illness of the GI tract and usually presents with a rapidly ascending, reversible muscle paralysis. Multiple cerebral infarcts would explain the upper motor signs but not the degeneration of lower motor neurons. Multiple sclerosis usually presents in middle-aged women with optic signs or singular neurologic defects that include peripheral sensory losses, not the universal loss of motor neurons described above.
79. C
This patient is having an immune complex (type III) hypersensitivity reaction to the tetanus booster commonly referred to as an Arthus reaction. Because the patient has been previously immunized, preformed circulating antibodies locally fixate complement at the injection site due to the high load of tetanus toxoid in the booster. This results in a local inflammatory vasculitis with pain, edema and skin ulceration.
Accumulation of mononuclear cells occurs in cell-mediated (type IV) hypersensitivity. Langerhans cells are dendritic cells of the skin that present foreign antigens in response to infection or foreign bodies, but antigen capture in the epidermis has no role in the Arthus reaction. Histamine release is a key component of immediate (type I) hypersensitivity. IgM and IgG proliferation occur in antibody-mediated (type II) hypersensitivity reactions.
(50% of users answered this question correctly) Edit Explanation 81. A
Inflammatory carcinoma of the breast creates pitting classically referred to as an "orange peel" appearance on clinical exam. It produces a rapid increase in breast size, itching, redness and warmth of the overlying skin. There is usually no obvious lump, and likewise inflammatory carcinoma is often confused with mastitis on clinical exam. The pitting is caused by severe inflammation destroying the suspensory (Cooper's) ligaments attaching breast tissue to the dermis, resulting in a dimpled "orange peel." The remaining answers do not play a part in the pathogenesis of pitting in inflammatory carcinoma.
(71% of users answered this question correctly) Edit Explanation 82. F
You need to know the likely organisms that cause osteomyelitis, and determine which of these are associated with sickle cell disease.
In general, Staph aureus and Salmonella enteritidis are the two most common causes of osteomyelitis in general. The vaso-occlusive bone disease related to sickle cell leaves the bone prone to infection. Here, the best answer choice is (F) Salmonella enteritidis.
(A) Clostridium family bugs cause Gas Gangrene, myonecrosis and septicemia, NOT osteomyelitis.
(B) Enterococcus faecalis is a Gram-Positive bacterium that normally inhabits the human gastrointestinal tract. It can exhibit antibiotic resistance and cause endocarditis and bladder or prostate infections.
(C) Listeria monocytogenes is a Gram positive, motile (with flagella) organism. It causes the disease Listerosis, which is characterized by septicemia, encephalitis and uterine/cervical infections.
(D) Proteus mirabilis is a Gram-negative, facultatively anaerobic bacterium. It shows swarming, motility, and urease activity. So, Proteus infection commonly results in struvite or calcium carbonate crystals in the urine, and to the formation of kidney stones.
83. E
This question tests basic anatomical knowledge of the nerves and muscles of the hand and their functions. Such questions are rare on the actual USMLE, but do exist.
The structure between the two tendons mentioned is the median nerve. Loss of the median nerve at the wrist results in loss of thenar muscle innervation, resulting in an inability to oppose the thumb.
(A) Adduction and abduction of the fingers are mediated by the palmar and dorsal interosseus muscles of the hand, respectively. These are mostly innervated by the ulnar nerve.
(B) Extension of the index finger requires forearm extensors.
(C) Flexion of fingers 4 and 5 requires forearm flexor muscles. This is a wrist injury.
(D) The ulnar nerve would mediate sensation over the base of the small finger.
(E) Opposition of the thumb and other fingers involves flexion of the flexor carpi radialis (FCR) and flexor digitorum superficialis (FDS), both innervated my the median nerve.
The FDS muscle in the forearm gives rise to 4 tendons that pass through the flexor retinaculum of the carpal tunnel and insert at the proximal interphalangeal joints. (So, it's an extrinsic muscle of the hand.)
The FCR muscle passes a tendon just lateral to the FC-ulnaris tendon, and inserts at the base of the 2nd metacarpal.
84. D
This question tests knowledge of the changes in fuel metabolism that occur with starvation.
Skeletal muscle oxidizes fuels to produce Acetyl CoA, which enters the citric acid cycle to yield ATP for energy during use. We need only to know which fuel provides the Acetyl CoA after 2 days of starvation. Let's go step by step.
In the resting state, skeletal muscles degrade adipose tissue triglycerides (C) and ketone bodies from the liver. During heavy bursts of activity, muscle uses phosphocreatine (A) stores and anaerobic respiration to break down muscle glycogen (B) to lactic acid. Moderately active muscle uses blood glucose (E) in addition to adipose-fatty acids and ketone bodies.
After an overnight fast, glycogen stores in the liver and muscle become depleted. After 24 hours, the blood glucose concentration begins to fall and glucagon is released.
After prolonged starvation (>24 hours), skeletal muscle uses circulating fatty acids (D) from triacylglycerol molecules that were broken down in adipose tissue. This permits the efficient use of fuel for muscle while reserving glucose (and later ketones) for the brain.
(63% of users answered this question correctly) Edit Explanation 85. B
Increased urinary excretion of catecholamines suggests increased production of catecholamines, a process that occurs in the adrenal medulla.
(You should know the various layers of the adrenal glands and the respective hormones produced in each layer.)
The patient presents with symptoms of hypertension secondary to a suprarenal mass, consistent with a pheochromocytoma. You should know that such masses are much more commonly benign (only 10% malignant).
Therefore (B) benign neoplasm of the adrenal medulla is the most likely cause.
(74% of users answered this question correctly) Edit Explanation 86. H
The key to answering this question is focussing on 4 key points:
1. diarrhea with or without blood
2. location: inflammatory changes are seen in the DISTAL part of the colon only
3. number of lesions: there are multiple ulcerations
4. depth of lesion: these ulcerations are limited to the mucosa.
Ulcerative colitis matches the characterization in the question stem along these 4 four points.
The various diseases listed in the answer choices can best be differentiated based on the 4 points listed above.
In particular, you must be familiar with the differences between Crohn's disease (B) and Ulcerative Colitis (E).
(A) AIDS associated gastroenteritis results in pathological changes more proximally than described here, and would not selectively target the colon. Futher, AIDS complications rarely result in inflammatory changes.
(B) Crohn's disease may occur anywhere throughout the colon and occurs in a patchy, discontinuous fashion. It penetrates deeper into the mucosa producing transmural fistulae and inflammation, unlike UC.
(C) C.difficile colitis (also called pseudomembranous colitis) typically affects older patients who have had prior exposure to antibiotics and rarely produces bloody diarrhea.
(D) E. coli is the most common cause of bacterial colitis, and depending on the pathogenic strain, may present differently. Notably, the lamina propria of the large intestine is infiltrated by polymorphonuclear leukocytes, without producing such a pattern of mucosal inflammation.
(89% of users answered this question correctly) Edit Explanation 87. D
This girl has severe carditis, which resulting enlargement of the cardiac shadow and acute congestive heart failure. This is a severe consequence of rheumatic fever, which is suggested by the acute-onset polyarthritis and rash (erythema marginatum). Rheumatic fever generally occurs 1-5 weeks after an upper respiratory infection caused by beta-hemolytic streptococcus. It is postulated that the immune system's response to the strep results in attack also on native tissues with similar antigens ("molecular mimicry").
Averting the possibility of rheumatic fever after streptococcus infection is the basis of penicillin treatment and prophylaxis for all people with suspected infection.
88. C
All the described abnormalities point to Cri-du-Chat syndrome, uniquely characterized by a 'crying of the cat' like vocalization made by affected infants. Cri-du-chat results from deletion of part of the short arm of chromosome 5, and spares the normal diploid number. This answer choice is 46,XX,del(5)(p12).
46,XX,del(5)(p12) describes a rare translocation that results in Acute Lymphoblastic Leukemia, not developmental disorders.
46,XX,del(5)(p12) indicates a trisomy in Chromosome 13, resulting in severe mental retardation and growth abnormalities called Patau's Syndrome. The description above suggests that this is not the best answer.
47,XX,+21 describes Trisomy in chr 21, which is classically Down's Syndrome. While Down's syndrome may result in microcephaly, it would most prominently cause mental retardation.
69,XXY is a total triploidy and usually incompatible with life.
89. D
The sudden onset of edema in the face and dependent limbs, coupled with hypoalbuminemia suggests a nephrotic syndrome in this child. Consistently, she has hyperlipidemia (cholesterol >200mg) and proteinuria. In children, the most common cause of a nephrotic syndrome is Minimal Change Disease (D, abbrev. MCD).
The child does not exhibit hypertension or overt hematuria. This eliminates renal diseases that are associated with a more NEPHRITIC picture, such as membranoproliferative glomerulonephritis (C) and Rapidly Progressive GN (E).
Be sure to review NEPHROTIC vs NEPHRITIC syndromes.
MCD, (aka lipoid nephrosis) involves T cell mediated destruction of the polyanion charge barrier on epithelial foot processes, which results in leakage of albumin into the urine. This is manifest in this patient as a reduced serum Albumin (<3.5mg) and Protenuria (>0.15g/24h). While MCD is often idiopathic, it has been described to occur days to weeks after upper respiratory infections, as here.
It is difficult to differentiate between MCD and the other nephrotic choices clinically, but you should know these important distinctions in their histology:
By definition, patients with MCD present histologically with normal looking glomeruli.
Focal segmental Glomerulosclerosis (A, FSGS) presents with focal sclerotic plaques of the glomerular tufts and hyaline deposits. It is also often associated with HIV and IV drug use.
Membranous glomerulonephritis (B) presents later in life, insidiously and is an immune complex disorder that results in mesangial deposits. The workup will show positive Anti nuclear antibodies and/or Anti dsDNA. (remember, this is a NEPHROPATHY and is assoc. with a NEPHROTIC picture despite its name).
(84% of users answered this question correctly) Edit Explanation 90. A
To answer this question, you must know which organelles are sources of calcium available for use WITHIN the cell.
The main sources of calcium in eukaryotic cells are the Smooth Endoplasmic Reticulum (A, SER) and the mitochondria (C).
Release of intracellular calcium stores upon G protein activation follows a well established pathway: the beta-gamma subunit of a G-protein is cleaved and proceeds to activate Phospholipase C (PLC). PLC, in turn, cleaves the membrane bound phospholipid [phosphatidylinositol 4,5-bisphosphate] (PIP2) molecule into 2 components: Diacylglycerol (DAG) and Inositoltriphosphate (IP3).
IP3 then binds to IP3 receptors present on the surface of the SER, which opens calcium channels and releases Ca2++ into the cytosol.
Calcium release by the mitochondria is not the direct target of the G-protein mediated calcium release.
The other answer choices are mere distractors and are NOT sources of calcium within animal cells.
(85% of users answered this question correctly) Edit Explanation |