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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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91. B
This girl has suffered from intussusception (choice B), when a piece of bowel gets “telescoped” into the proximal bowel, leading to ischemia and infarction. The patient’s abdominal pain and vomiting are consistent with this diagnosis. The picture is key to this question, since all of the other choices could have similar clinical presentations.
Appendicitis (choice A) would also present with RLQ pain and vomiting but would not involve the adjacent bowel.
Meckel diverticulum (choice C) would present similarly to appendicitis but the would show an outpouching of bowel, not an entire infarcted region.
Necrotizing enterocolitis (choice D) is unusual in a four year old; most patients are newborns.
Strangulated hernia (choice E), would present similarly but the pathology would not show one piece of bowel telescoped into the other.
(60% of users answered this question correctly) Edit Explanation 92. D
The question stem provides 3 useful hints which all point to Neutrophils (D) as the answer.
First, the boy's infections are of pyogenic origin, indicating bacterial infection that results in pus formation. Pus consists primarily of expended neutrophils. Since there is a defect in the pyogenic response (resulting in repeated infection), there is most likely a defect in the action of Neutrophils.
Second, his normal antibody response to immunizations can help you rule out B Lymphocytes (A) or Macrophages (C, the most common antigen presenting cell type) as the answers.
Third, a normal recovery from chickenpox and measles (both viruses) requires intact function of T lymphocytes (E) and Eosinophils (B, because they contain abundant RNAses).
The other answers do not play a direct role in the body's response to pyogenic infections.
(71% of users answered this question correctly) Edit Explanation 93. C
Normal grief reaction, an emotional response to loss lasting up to 6 months, is characterized by feelings of physical pain, distress, and physical and emotional suffering. It may include symptoms such as diminished appetite, difficulty sleeping, restlessness, anhedonia, auditory and visual hallucinations, and feelings of guilt.
Although patients suffering from a normal grief reaction may be depressed, the diagnosis of major depressive disorder is reserved for a small subset with generalized feelings of hopelessness, helplessness, worthlessness, and guilt that persists for more than 6 months.
Dysthymic disorder has some features of major depressive disorder (anhedonia, sleep disturbance, etc.) but usually does not last as long and is not as disabling.
Obsessive-compulsive disorder is a mood disorder that involves distressing, intrusive thoughts (obsessions), and the ritualized repetitive behaviors (compulsions) sometimes employed to ward them off.
Schizoaffective disorder comprises features of both schizophrenia and a mood disorder, and may include delusions, disorganized thinking and/or speech, and manic and/or depressive episodes.
(92% of users answered this question correctly) Edit Explanation 94. A
Presbycusis, or age-related sensorineural hearing loss, affects a majority of the elderly. Hearing loss is usually symmetrical, and is most prominent in high-frequency tones. Diminished perception of any of the other tones in the list suggests a pathological condition and warrants further work-up.
(78% of users answered this question correctly) Edit Explanation 95. B
Increased carbon dioxide shifts the oxygen-dissociation curve to the right, leading to an increase in oxygen unloading. Curve B is the only option that depicts this right-shift while maintaining the shape of the curve. Other factors you should know that can shift the curve to the right include higher temperature, increased 2,3-BPG, and lower pH.
(77% of users answered this question correctly) Edit Explanation 96. C
This patient may be developing rhabdomyolysis - a very serious complication involving muscle necrosis and release of cellular contents into the bloodstream. Clinical signs include diffuse muscle pain, increased serum creatine kinase, elevated serum potassium, and acute renal failure - indicated by a rapidly rising creatinine level. Although rhabdomyolysis is a rare side effect, it has been associated with the administration of statins, such as lovastatin, especially when taken in combination with cyclosporine or fibrates. None of the other drugs listed above has been associated with rhabdomyolysis.
(87% of users answered this question correctly) Edit Explanation 97. D
This patient has been poisoned by the organophosphate parathion. Organophosphates inhibit acetylcholinesterase (AChE), leading to toxic build-up of acetylcholine (ACh) and resulting in the symptoms listed above. Remember the mnemonic DUMBELS associated with cholinergic toxicity: Defecatioin, Urination, Miosis, Bronchorrhea/Bronchospasm/Bradycardia, Emesis, Lacrimation, Salivation.
(68% of users answered this question correctly) Edit Explanation 98. C
To answer this question, you need to know that pain and temperature sensation are carried in the spinothalamic tract. Therefore, impaired pain and temperature sensation points to a lesion in the contralateral spinothalamic tract (Region C). Specifically, the spinothalamic tract carries information about pain and temperature from the contralateral side of the body from the dorsal root ganglion, crossing at the anterior white commissure and finally synapsing in the VPL thalamic nucleus.
Region A is the hypoglossal nerve (cranial nerve XII), which controls the tongue muscles.
Region B is the trigeminal nerve (cranial nerve V) which carries pain and temperature from the ipsilateral face.
Region D shows damage to the medullary pyramid which controls the skeletal muscles of the contralateral body.
Region E is the medial lemniscus, which controls proprioception and fine touch of the contralateral body.
(63% of users answered this question correctly) Edit Explanation 99. B
This patient has drug-induced lupus. The malar rash, joint tenderness, and pericardial friction rub are characteristics of the disease, and are often associated with a dramatic rise in anti-nuclear antibodies (ANA). Although only 5-10 percent of patients develop clinical signs, hydralazine is known to induce auto-antibodies - including ANA - in a significant number of people. Long term treatment with procainamide has been associated with even higher rates of drug-induced lupus. Other drugs that can cause a lupus-like reaction are minocycline and anti-TNF agents.
(83% of users answered this question correctly) Edit Explanation 100. A
Theophylline is metabolized in the liver primarily by isoforms 1A2 and 3A4 of cytochrome P450 (CYP1A2 and CYP3A4). Inhibitors of these isoforms lead to slower metabolism of theophylline, which can result in toxic plasma concentrations if the dose of theophylline is not reduced appropriately. This question is asking you to identify which of the drugs in the list inhibits CYP1A2 and CYP3A4. Cimetidine is the only drug that is a major inhibitor of both of these isoforms.
(75% of users answered this question correctly) Edit Explanation 101. A
Small, atretic ovarian follicles are a normal finding in post-menopausal women. As women enter menopause, their ovaries lose the ability to support viable ova and follicles. This process of planned cell death is called apoptosis. By contrast, necrosis is unplanned cell death caused by an insult to previously viable tissue. Metaplasia and transformation are associated with neoplastic processes, which may be occurring in this woman's uterus leading to bleeding. Don't be fooled, the question asks not about uterine histological findings, but rather about those in the ovaries which are entirely benign.
(79% of users answered this question correctly) Edit Explanation 102. D 103. A
A warm, painful, and erythematous breast mass (often accompanied by fluctuance), experienced by breast-feeding women, that resolves with antibiotics suggests an infectious cause. Although far less common than mastitis, a breast abscess is the only infectious etiology among the answer choices. Risk factors include primiparity, maternal age >30, and a pregnancy lasting longer than 41 weeks. Fibrosis (scarring) can replace the abscess in some women, leaving a firm mass and dimpled overlying skin months after the abscess has resolved.
(71% of users answered this question correctly) Edit Explanation 104. B
This man is exhibiting the classic signs of (B) delirium: an acute change in mental status, inattention, disorganized thinking, and a fluctuating level of awareness. Delirium can be caused by any of the following in the mnemonic P. DIMM WIT: Postoperative state, Dehydration/malnutrition, Infection, Medications, Metals (heavy metals exposure), Withdrawal from alcohol/drugs, Inflammation, and, as in this case, Trauma/Burns.
The acute onset of this change in mental status makes (B) delirium a much more likely diagnosis than (A) adjustment disorder, (C) Paranoid personality disorder, (D) Schizophrenia, or (E) Alzheimer's dementia.
(84% of users answered this question correctly) Edit Explanation 105. B
A high spiking fever and shaking chills after a trip to central Africa is a classical story for malaria of the Plasmodium genus. The conventional method for diagnosing malaria is a (B) Giemsa-stained blood smear.
(A) Acid-fast stains are classically used for diagnosis of tuberculosis, while (D) India ink wet mount is known for its use with Cryptococcus neoformans. (E) KOH stain is used for fungal elements in thick mucoid material and in specimens containing keratinous material, such as skin scales, nails, or hair.
(74% of users answered this question correctly) Edit Explanation 106. A
The commonly-used HIV medications have well-known side effects. In particular, zidovudine (AZT), the commonly-used nucleoside reverse transcriptase inhibitor (NRTI), may cause anemia through (A) decreased formation of erythrocytes. Approximately 1% of patients experience this adverse effect after 2-4 weeks of taking the medication. Other significant side effects of AZT are headache (63%), malaise (53%), nausea (51%), anorexia (20%), vomiting (17%), constipation (6%), and granulocytopenia (2%), anemia (1%).
(B) Folic acid deficiency and (E) iron deficiency may, respectively, cause macrocytic and microcytic anemia, but they are not in and of themselves related to HIV medications. Similarly, these medications do not affect (C) the formation of erythrocyte antibodies or (D) the fragility of erythrocytes.
(72% of users answered this question correctly) Edit Explanation 107. F
To answer this question, you need to know that beta-thalassemia major is caused by under-production of the beta globin protein, the product of the beta-globin gene. Since all three of the nucleotides in this question are upstream of the transcription start site, they are not part of the gene transcribed, and therefore are not required for binding elongation factors, such as eIF4F (choice A) or eIF4G (choice B), which are involved in translation of proteins, not the transcription.
Similarly, histone acetylase and deacetylase (choices C and D) are involved in chromatin packing of the gene as a whole, not a particular region, as in this example.
A loss of function mutation in positive-acting transcription factors (choice F) is the only choice that would result in under-production of the entire protein product by affecting the promotor region.
108. B
Severe combined (T and B) immunodeficiency, also known as SCID, is caused a purine salvage pathway deficiency of (B) adenosine deaminase. Without this enzyme to convert adenosine to inosine, there is an excess of ATP, dATP, and S-adenosylhomocysteine. These substances are toxic to immature lymphoid cells, and also prevent DNA synthesis through feedback inhibition of ribonucleotide reductase. Therefore, lymphoid cells fail either to reach maturity or to be produced in the first place, resulting in SCID.
Deficiency of (A) adenine phosphoriboxyltransferase (APRT) may lead to kidney stones formed of adenine and salts.
Deficiency of (C) adenosine kinase is associated with nonspecific developmental abnormalities.
(D) Adenylosuccinate synthetase deficiency has been implicated in the development of hyperuricemia and gout.
Deficiency of (E) Hypoxanthine-guanine phosphoribosyltransferase causes Lesch-Nyhan Syndrome, characterized by self-mutilation, retardation, aggression, hyperuricemia, and gout.
(76% of users answered this question correctly) Edit Explanation 109. E
In this patient with a history of ovarian cancer, the most likely cause of her edema is (E) obstruction of lymph vessels secondary to her malignancy. While various clinical conditions can cause edema, including congestive heart failure, cirrhosis, and nephrotic syndrome, these disorders are typically associated with pitting edema. This patient presents with nonpitting lymphedema, characterized by its involvement of an isolated limb.
(A) Decreased capillary hydrostatic pressure would not cause edema, although increased capillary hydrostatic pressure would.
(B) Decreased interstitial hydrostatic pressure is unlikely.
(C) Increased capillary oncotic pressure would not cause edema, although decreased capillary oncotic pressure would.
(D) Increased capillary permeability, as in seen in burns, trauma, inflammation, sepsis, and allergic reactions, may cause edema. However, this presentation makes lymphedema much more likely.
(84% of users answered this question correctly) Edit Explanation 110. E
Rhabdomyolysis (Choice E) occurs when muscle fibers break down, often due to stress (such as running a race), releasing myoglobin into the bloodstream, which can then damage the kidneys. The recent history of a race, as well as the urinalysis which is positive for blood on dipstick but negative for RBCs supports this diagnosis.
Acute glomerulonephritis (Choice A) usually results in red blood cells in the urine and is unlikely with no history of infection or other symptoms.
Hypovolemia (Choice B) can lead to acute renal failure by prerenal mechanism but would not cause a positive dipstick for blood.
Renal infarct (Choice C) would present with severe pain and renal failure, as would renal vein thrombosis (Choice D).
111. A
Doxorubicin has a well known side-effect profile of (A) dilated cardiomyopathy, with dose-related CHF and transient ECG abnormalities occuring in over 10% of patients. Other side effects include leukopenia (75%) nausea and vomiting (21-55%). It is not as closely associated with restrictive cardiomyopathy or pulmonary disease. The chemotherapeutic agent bleomycin is better known for causing pulmonary toxicity, particularly pulmonary fibrosis.
113. D
Immune thrombocytopenic purpura (ITP) (D) causes a profound decrease in platelets, can reveal petechiae on physical exam, and should be of rapid onset with all other laboratory values normal. Bone marrow biopsy would reveal normal-to-increased numbers of megakaryocytes.
Marrow biopsy in acute megakaryocytic leukemia (A) would show hypercellular marrow replaced by blast cells and dysmorphic promegakaryocytes and megakaryocytes. This patient has mature megakaryocytes.
In acute myelogenous leukemia (B), blast red cells should be seen in the peripheral smear and bone marrow. This patient has normal red cell morphology.
In aplastic anemia (C) there are profound decreases in red cells, white cells, and platelets. This patient has normal RBC and WBC values.
Epstein-Barr (EBV) viral infection (E) is usually associated with a leukocytosis and mild thrombocytopenia, not the normal WBC count and profound platelet deficiency seen here. Though immune thrombocytopenic purpura is a rare complication of EBV, (D) is a better answer choice in this situation.
(82% of users answered this question correctly) Edit Explanation 114. B
Acute physical or psychological stress activates the hypothalamic-pituitary-adrenal axis, resulting in increased plasma ACTH and (B) cortisol concentrations. Fever, caused by systemic infection or by pyrogen administration, is a potent stimulus of ACTH and cortisol secretion, while infection causes release of interleukins IL-1 and IL-6, which stimulate hypothalamic CRH secretion, and tumor necrosis factor-alpha, which stimulates ACTH secretion directly.
Cortisol, in turn, acts as a physiological antagonist to insulin by inhibiting glycogenesis, promoting lipolysis, and mobilizing extrahepatic amino acids and ketone bodies. This leads to gluconeogenesis with resultant increased circulating glucose concentrations.
(75% of users answered this question correctly) Edit Explanation 115. D
Quadrant D (D) contains CD4+ T lymphocytes (a.k.a “helper” T cells). This population is activated by antigen presented on major histocompatibility complex (MHC) class II molecules, which are only expressed on “professional” antigen presenting cells (APCs). These antigens are fragments of nonself peptides internalized by APCs. This CD4+ population would respond to hepatitis A virus vaccine.
Quadrant A (A) contains CD8+ T lymphocytes (a.k.a. “cytotoxic” T cells). This population is activated by antigen presented on major histocompatibility complex (MHC) class I molecules, which are expressed on nearly all nucleated cells. Such antigens are selected at random from cytosol content, and may be self- or nonself-peptides (e.g. infectious virion). This CD8+ population would respond to virulent hepatitis A virus, but not the vaccine.
Quadrant B (B) contains CD4+ CD8+ T lymphocytes, corresponding to an intrathymic (immature) phenotype. Such cells are usually not seen peripherally, and do not express mature T cell function.
Quadrant C (C) would contain CD4- CD8- T lymphocytes. Such cells would be abnormal and would not be involved in a vaccine response.
(70% of users answered this question correctly) Edit Explanation 116. E
A connexon is an assembly of 6 proteins (called connexins) that forms a gap junction in the (E) transmembrane region between the cytoplasm of two adjacent cells. The connexon is actually the hemichannel supplied by a cell on one side of the junction; two connexons from opposing cells normally come together to form the complete intercellular gap junction channel. However, in some cells, the hemichannel itself is active as a conduit between the cytoplasm and the extracellular space.
(72% of users answered this question correctly) Edit Explanation 118. B
Prolonged bleeding times are commonly seen in infants who have a deficiency of Vitamin K. The sterile intestines of newborns are lacking in intestinal flora to synthesize Vitamin K, and this newborn is at particular risk, having recently undergone an antibiotic regimen. Therefore, she is likely deficient in Vitamin K. Vitamin K is necessary for the (B) γ (gamma) carboxylation of glutamic acid residues of proteins necessary for blood clotting. Vitamin K-dependent clotting factors II, VII, IX, and X are therefore deficient, resulting in prolonged PT and aPTT times. Anticoagulant Proteins C and S are also affected by the lack of Vitamin K, but the hypocoagulability is more significant in Vitamin K deficiency.
(69% of users answered this question correctly) Edit Explanation 119. E
Proteinuria, edema, and hypertension after 20 weeks' gestation in a previously normotensive woman are pathognomonic for (E) preeclampsia. Preeclampsia occurs in approximately 5 to 8 percent in the United States, although it is mild in 75 percent of cases. This disease is associated with the HELLP syndrome (Hemolysis, Elevated LFTs, Low Platelets.) When the above presentation includes seizures, a diagnosis of (C) eclampsia is made. In this case, this woman's headaches are caused by hypertension, while her edema is secondary to the reduced plasma oncotic pressure following protein loss in the urine.
(A) Acute glomerulonephritis can cause hypertension, but is less likely to cause edema, while (D) Nephrotic syndrome causes edema without hypertension. (B) Congestive heart failure does not cause orbital edema.
120. B
The lab values for hemotocrit and hemoglobin indicate that this women has anemia. Her MCV value of 70 is consistent with a microcytic anemia. The most common microcytic anemia is iron deficiency anemia. Chronic blood loss through the GI tract is the most common cause of iron deficiency anemia in men and post menopausal women.
An acquired hemolytic anemia likely would present with additional symptoms on physical exam, including signs of jaundice, dark urine upon standing, and splenomegaly. Also, the history does not suggest a cause for hemolysis.
Beta-Thalassmia minor is a mild microcytic anemia. In the majority of cases, Beta-Thalassemia minor is asymptomatic, and many affected people are unaware of the disorder. This is an inherited anemia that would not present late in life with increasing shortness of breath.
Folic acid deficiency and Pernicious anemia leading to Vitamin B12 deficiency are both types of macrocytic and megaloblastic anemia. The MCV value would be elevated above 96 micrometer^3 and these types of anemia are less common than iron deficiency anemia.
(77% of users answered this question correctly) Edit Explanation |