USMLE Step 1 练习题

The esophagus has three normal points of narrowing: at the cricopharyngeus (C6), at the aortic arch and left main bronchus (T4-T5), and at the diaphragm (T10). This patient's indentation is at the aortic arch level, so the arch itself is responsible. An indentation lower in the mediastinum would more specifically implicate the left main bronchus.

From lateral to medial, the femoral triangle contains the femoral nerve, femoral artery, femoral vein, and empty space, summarized as NAVEL. The femoral vein therefore lies immediately medial to the artery. The femoral nerve is lateral, and the obturator nerve is not a content of the femoral triangle.

Zone 3 is farthest from the portal blood supply and is the primary site of CYP2E1-mediated alcohol metabolism, making it most vulnerable to ischemia and to alcohol and acetaminophen toxicity. Zone 1 receives the most oxygenated blood and is preferentially affected in conditions such as toxemia of pregnancy. Centrilobular necrosis with Mallory bodies is therefore the expected finding.

Erb-Duchenne palsy results from traction on the upper trunk (C5-C6), often during shoulder dystocia. Loss of the supraspinatus, deltoid, and biceps produces the waiter's tip posture. Klumpke palsy of the lower trunk instead affects intrinsic hand muscles and may cause Horner syndrome.

The radial nerve innervates the wrist and finger extensors, so injury at the radial groove causes wrist drop. Ulnar nerve injury paralyzes most intrinsic hand muscles, producing clawing of the ring and little fingers. Median nerve injury causes ape hand from thenar muscle paralysis, and axillary nerve injury paralyzes the deltoid, limiting abduction to the first 15 degrees performed by the supraspinatus.

The low pH and low bicarbonate indicate metabolic acidosis. The low pCO2 reflects compensatory hyperventilation rather than a primary respiratory disorder. Winter's formula (expected pCO2 = 1.5 x HCO3 + 8 +/- 2) predicts a pCO2 near 26 mm Hg, consistent with appropriate respiratory compensation.

Inulin clearance is the classic measure of GFR because inulin is freely filtered and inert in the tubule. PAH clearance estimates renal plasma flow because PAH is also secreted. Filtration fraction is calculated as GFR divided by renal plasma flow.

Classic galactosemia is caused by deficiency of galactose-1-phosphate uridyltransferase and presents in neonates fed milk with liver disease, cataracts, and E. coli sepsis. Galactokinase deficiency causes cataracts without the systemic liver toxicity. Treatment is dietary galactose restriction.

The plateau is maintained by inward calcium current through L-type calcium channels that is balanced by outward delayed rectifier potassium current. Fast sodium channels open during phase 0 and are inactivated before the plateau. The sodium-potassium ATPase gradually restores ionic gradients rather than generating the plateau.

2,3-BPG binds deoxyhemoglobin and stabilizes the T (low-affinity) state, shifting the oxygen-hemoglobin dissociation curve to the right. This decreases oxygen affinity and enhances oxygen unloading in tissues. Chronic hypoxia and anemia raise 2,3-BPG levels.

Nonselective beta blockers antagonize beta-2 receptors in bronchial smooth muscle, which can trigger bronchospasm in patients with asthma. Beta-1 selective agents are relatively safer but can lose selectivity at high doses. ACE inhibitors cause cough through bradykinin but do not directly cause bronchoconstriction.

Chloramphenicol binds the 50S ribosomal subunit and inhibits peptidyl transferase, blocking peptide bond formation. Tetracycline binds the 30S subunit and blocks tRNA binding. Vancomycin inhibits cell wall peptidoglycan synthesis, and sulfamethoxazole inhibits folate synthesis.

X-linked agammaglobulinemia is caused by BTK mutations that arrest B-cell development, producing absent B cells and panhypogammaglobulinemia with intact T-cell immunity. CD40 ligand deficiency causes hyper-IgM syndrome, and JAK3 or RAG1 defects cause severe combined immunodeficiency with impaired T-cell function.

Protamine sulfate is a positively charged protein that binds heparin electrostatically and neutralizes it. Vitamin K reverses warfarin, not heparin. N-acetylcysteine is used for acetaminophen toxicity, and flumazenil reverses benzodiazepines.

ACE normally degrades bradykinin, so ACE inhibition raises bradykinin levels, causing dry cough and angioedema; these reactions are not IgE-mediated. In bilateral renal artery stenosis, loss of angiotensin II-mediated efferent arteriolar constriction can precipitate acute kidney injury. AT1 receptor blockade is the mechanism of angiotensin receptor blockers, not ACE inhibitors.

Primary syphilis is caused by Treponema pallidum and classically presents with a painless chancre with a firm, rolled border and painless regional adenopathy; dark-field microscopy shows spirochetes. Haemophilus ducreyi causes painful chancroid ulcers. Chlamydia and gonorrhea typically cause urethritis or cervicitis rather than a genital ulcer.

Mycoplasma pneumoniae causes atypical walking pneumonia, frequently with cold agglutinins, and lacks a cell wall, explaining the poor response to beta-lactams such as amoxicillin. Streptococcus pneumoniae would appear as gram-positive diplococci but responds to beta-lactams. Heterophile antibodies suggest infectious mononucleosis, which does not produce this pattern of pneumonia.

Staphylococcus aureus is the most common cause of native valve infective endocarditis, especially in injection drug users, and characteristically involves the tricuspid valve, producing a pansystolic murmur. Viridans streptococci are catalase negative and classically follow dental procedures. Coxiella burnetii is a cause of culture-negative endocarditis.

Toxoplasma encephalitis is a common opportunistic infection at CD4 counts below 100 and classically produces multiple ring-enhancing brain lesions, often in the basal ganglia. Cryptococcus neoformans typically causes meningitis without ring-enhancing masses. JC virus causes progressive multifocal leukoencephalopathy with nonenhancing white-matter lesions.

Clostridioides difficile is an anaerobic spore-forming Gram-positive rod that causes antibiotic-associated colitis through toxins A and B. Transmission is primarily person-to-person through spores in health care settings rather than contaminated food. For mild disease, stopping the inciting antibiotic may suffice, and more severe cases are treated with metronidazole or oral vancomycin.

Aromatic amines such as benzidine and beta-naphthylamine are associated with urothelial carcinoma of the bladder. Asbestos is linked to mesothelioma and lung cancer, vinyl chloride to hepatic angiosarcoma, and radon to lung cancer.

Invasive lobular carcinoma of the breast is characterized by loss of E-cadherin, which produces dyscohesive cells with the Indian file growth pattern and a higher rate of bilaterality. HER2 amplification is more common in high-grade ductal carcinomas. BRCA1 mutations increase breast cancer risk but do not define this histologic pattern.

Yolk sac (endodermal sinus) tumor secretes alpha-fetoprotein and is the most common malignant germ cell tumor in prepubertal boys; it can also appear in mixed adult germ cell tumors. Pure seminoma does not produce AFP and only occasionally produces hCG. Choriocarcinoma secretes hCG, and Leydig cell tumors may produce androgens or estrogens.

HBV can integrate into host DNA and causes chronic hepatitis and cirrhosis, all of which promote hepatocellular carcinoma. HBV-related HCC can occur even without cirrhosis, and vaccination reduces this risk. Chronic HCV infection also increases HCC risk, mainly through cirrhosis.

Classic Reed-Sternberg cells are CD15-positive and CD30-positive, weakly express or lack CD20, and are clonal germinal center B cells that have lost much of the B-cell program. EBV is found in many cases, especially mixed cellularity subtype. The surrounding node contains a characteristic mixed inflammatory infiltrate that includes eosinophils.

Normal bereavement may include sadness, insomnia, and reduced interest, but pervasive worthlessness, psychomotor retardation, and suicidal ideation point to major depressive disorder. Sadness in waves triggered by reminders is typical of uncomplicated grief. Distinguishing the two matters because major depressive disorder is treated with antidepressants and psychotherapy.

Lithium, valproate, and atypical antipsychotics are first-line agents for acute mania and long-term mood stabilization. Antidepressants such as fluoxetine and bupropion can precipitate or worsen manic episodes. Buspirone is an anxiolytic with no role in acute mania.

Dopaminergic therapy, especially levodopa and dopamine agonists, can cause hallucinations and psychosis, particularly in older patients and those with dementia. Donepezil is used for Alzheimer disease, divalproex is a mood stabilizer, and sertraline is an antidepressant. Reduction of dopaminergic medication is often attempted before adding antipsychotics.

Respect for autonomy requires honoring the informed decisions of a capacitated patient, including refusal of life-sustaining treatment. Beneficence and nonmaleficence explain why clinicians recommend treatment but do not override valid refusal. Justice concerns fair distribution of resources and does not directly govern this decision.

Informed consent requires that a capacitated patient voluntarily consent after receiving information about the procedure, its risks, benefits, and reasonable alternatives. Exceptions include emergencies in which the patient lacks capacity and no surrogate is available. Information must be tailored to what a reasonable patient would want to know.

Fragile X syndrome is caused by CGG repeat expansion in FMR1 and is the most common inherited cause of intellectual disability, with a long face, large ears, and macro-orchidism in males. Klinefelter syndrome (47,XXY) causes tall stature and small testes, and Down syndrome has distinct facial features and congenital heart disease. Turner syndrome affects females.

Prader-Willi syndrome results from loss of paternally expressed genes on 15q11-q13, usually by paternal deletion, and presents with neonatal hypotonia, poor feeding, and later hyperphagia and obesity. Angelman syndrome occurs when the maternal copy is lost and causes severe developmental delay, seizures, and inappropriate laughter. Beckwith-Wiedemann syndrome involves overgrowth and imprinting abnormalities on chromosome 11.

A 45,X karyotype represents monosomy for the X chromosome and is among the most common chromosomal causes of early pregnancy loss. Most 45,X conceptuses abort spontaneously; surviving females have short stature, gonadal dysgenesis, and often cardiac or renal anomalies. Trisomy 21 would appear as 47,XX or 47,XY with an extra chromosome 21.

In autosomal recessive inheritance, two carriers have a 25% risk of an affected child, a 50% risk of a carrier, and a 25% risk of an unaffected noncarrier for each pregnancy. Each pregnancy is independent, and there is no sex predilection. Because rare recessive alleles are often identical by descent, consanguinity increases the risk.

HELLP syndrome is a severe variant of preeclampsia defined by hemolysis, elevated liver enzymes, and low platelets. Eclampsia is preeclampsia complicated by seizures. Placenta previa typically causes painless bleeding because the placenta covers the cervical os, whereas abruption causes painful bleeding from premature placental separation; both require prompt obstetric evaluation.

A tumor at the lung apex can invade the sympathetic chain, causing ipsilateral ptosis, miosis, and anhidrosis (Horner syndrome) together with shoulder and arm pain; this combination is called Pancoast syndrome. Superior vena cava syndrome causes facial and upper extremity swelling with distended neck veins. Lambert-Eaton myasthenic syndrome is associated with proximal weakness and autonomic symptoms, not a Horner triad.

Elevated TSH with low free T4 is the classic pattern of primary hypothyroidism, which presents with fatigue, weight gain, cold intolerance, constipation, and dry skin. Hyperthyroidism causes weight loss, heat intolerance, and diarrhea. Subclinical hyperthyroidism has a low TSH with normal free T4, and sick euthyroid syndrome typically occurs in acute illness with normal or altered TSH and thyroid hormone levels.

First-line eradication of H. pylori uses triple therapy with a proton pump inhibitor plus two antibiotics, commonly amoxicillin and clarithromycin. This regimen improves ulcer healing and reduces recurrence. Aspirin can worsen peptic ulcer disease, corticosteroids and sucralfate alone do not eradicate the infection, and antibiotic monotherapy has unacceptably high resistance and failure rates.

Pheochromocytoma arises from chromaffin cells of the adrenal medulla and causes episodic catecholamine excess with palpitations, diaphoresis, headache, and hypertension; elevated urinary metanephrines support the diagnosis. Adrenocortical adenomas cause cortisol or aldosterone excess, carcinoid tumors produce serotonin-related symptoms, and pituitary adenomas can cause hormonal syndromes but not this catecholamine pattern.

The sudden onset of a severe thunderclap headache with meningismus and xanthochromic cerebrospinal fluid strongly suggests subarachnoid hemorrhage, most often from a ruptured cerebral aneurysm. Xanthochromia results from red blood cell breakdown and is an important finding that can persist for days after bleeding, unlike a traumatic lumbar puncture.

The L4 nerve root supplies the tibialis anterior, which dorsiflexes and inverts the foot. The L5 root contributes to the extensor hallucis longus and foot eversion, while S1 supplies the gastrocnemius and plantarflexion. An L4 radiculopathy therefore fits the described deficit.

Interruption of the sympathetic pathway produces Horner syndrome: ptosis, miosis, and anhidrosis. A CN III lesion causes a dilated pupil and an eye that is down and out, whereas facial nerve damage causes weakness of facial expression. Trigeminal involvement would produce facial sensory loss or masticatory weakness rather than this triad.

The lesser sciatic foramen transmits the pudendal nerve, internal pudendal vessels, and the tendon of obturator internus. The sciatic nerve and superior gluteal structures leave the pelvis through the greater sciatic foramen. The obturator nerve passes through the obturator canal.

The median nerve supplies cutaneous sensation to the lateral palm and the palmar tips of the lateral 3.5 digits. The ulnar nerve supplies the medial 1.5 digits, and the radial nerve supplies the dorsolateral hand. The musculocutaneous nerve is primarily a lateral forearm sensory nerve.

The right recurrent laryngeal nerve passes posterior to the right subclavian artery, while the left passes under the aortic arch. It supplies the intrinsic laryngeal muscles except the cricothyroid, which is innervated by the external branch of the superior laryngeal nerve. The statement is therefore correct.

Hyperventilation lowers arterial PCO2 and raises pH, producing respiratory alkalosis. The lightheadedness reflects hypocapnia-induced cerebral vasoconstriction and reduced cerebral blood flow. Slow breathing with rebreathing allows PCO2 to normalize.

Cardiac output equals heart rate multiplied by stroke volume: 48 x 100 mL = 4800 mL/min, or 4.8 L/min. The trained heart maintains a normal resting cardiac output through increased stroke volume despite bradycardia. Values of 4 to 8 L/min are typical for a resting adult.

Efferent arteriolar constriction raises glomerular capillary hydrostatic pressure, increasing GFR while reducing renal plasma flow. The filtration fraction therefore increases. This mechanism explains why ACE inhibitors reduce GFR in patients with efferent arteriolar constriction from angiotensin II.

The elevated PCO2 with acidemia establishes respiratory acidosis. The bicarbonate is substantially increased, which indicates chronic renal compensation rather than an acute process. A primary metabolic alkalosis is not needed because the compensation is appropriate for a chronic respiratory acidosis.

These conditions decrease hemoglobin's affinity for oxygen, shifting the curve right and increasing P50. A right shift favors oxygen unloading in exercising or acidotic tissue. The statement is correct.

Hydralazine is a well-known cause of drug-induced lupus, especially in slow acetylators, and is often associated with anti-histone antibodies. Lisinopril can cause cough or angioedema, metoprolol can cause bradycardia, and thiazides cause electrolyte disturbances rather than a lupus syndrome.

Beta-blockers prolong AV nodal refractoriness and slow the ventricular response in atrial fibrillation. Loop diuretics, ACE inhibitors, and alpha-1 blockers do not directly slow AV nodal conduction. Calcium channel blockers and digoxin are alternatives for rate control.

Atropine antagonizes muscarinic receptors and reverses bradycardia, salivation, and diarrhea from cholinesterase inhibitor excess. Pralidoxime reactivates inhibited acetylcholinesterase but does not directly reverse existing muscarinic symptoms. Naloxone and flumazenil are opioid and benzodiazepine antagonists, respectively.

Methotrexate inhibits dihydrofolate reductase, depleting reduced folate needed for DNA synthesis. Supplemental folic acid reduces stomatitis, cytopenias, and other toxicities without eliminating efficacy. Vitamin K, vitamin C, and iron do not correct this mechanism.

Bruton tyrosine kinase is required for pre-B cell maturation, so patients have absent B cells and very low immunoglobulins. Maternal IgG protects during the first months of life, so bacterial infections typically begin around 6 months. T-cell function is intact, so opportunistic infections are not a prominent feature.

Campylobacter jejuni is a curved gram-negative rod that commonly causes diarrhea after consumption of undercooked poultry. Shigella causes bloody mucoid dysentery with a low infectious dose, Vibrio cholerae is linked to seafood and produces rice-water diarrhea, and amebiasis is associated with invasive large-bowel disease.

Clostridium perfringens causes gas gangrene with myonecrosis, gas production, and foul-smelling exudate after contaminated wounds. It appears as large gram-positive rods with spores. Anthrax follows contact with animal products, Nocardia is weakly acid-fast, and Actinomyces produces sulfur granules.

Candida esophagitis classically appears as white plaques and is common when the CD4 count is very low. CMV causes large, deep ulcers and HSV causes small punched-out ulcers, both less consistent with scraping white plaques. Fluconazole is the usual treatment for candidal esophagitis.

Toxoplasmosis is the most common cause of multiple ring-enhancing brain lesions in advanced AIDS and is treated with pyrimethamine plus sulfadiazine. Cryptococcus typically causes meningitis rather than focal lesions, Pneumocystis causes pneumonia, and MAC causes disseminated infection with fever and weight loss.

Parvovirus B19 infects erythroid progenitors, causing transient aplastic crisis in patients with hemolysis and fifth disease in children. Fetal infection can suppress erythropoiesis and cause hydrops fetalis. It does not target hepatocytes, and no routine parvovirus vaccine is available.

Small cell lung carcinoma frequently secretes ADH or ectopic ACTH, causing SIADH or Cushing syndrome. Hypercalcemia from PTHrP is typical of squamous cell carcinoma, gynecomastia suggests hCG-producing germ cell tumors, and acanthosis nigricans is associated with gastrointestinal adenocarcinomas.

Clear cell carcinoma of the ovary is strongly associated with endometriosis and shows clear glycogen-rich cells. Serous carcinoma is the most common ovarian malignancy but does not have this clear-cell appearance. Mucinous tumors contain mucin-secreting glands, and mature teratomas are benign with differentiated tissues.

Painless jaundice with a palpable gallbladder is Courvoisier sign, classically caused by adenocarcinoma of the pancreatic head obstructing the distal bile duct. Most pancreatic ductal adenocarcinomas are unresectable at diagnosis. Neuroendocrine tumors produce hormones, and serous cystadenomas are benign.

Prostate adenocarcinoma characteristically metastasizes to bone, especially the axial skeleton, and produces osteoblastic lesions. Bone pain and pathologic fractures are common late complications. Visceral metastases to brain, spleen, or bowel are much less typical.

Barrett esophagus is defined by intestinal metaplasia containing goblet cells in the distal esophagus. It is a precursor of esophageal adenocarcinoma, so periodic surveillance endoscopy is recommended. Progression is not inevitable and may take years.

Recurrent unexpected panic attacks with anticipatory worry and avoidance are characteristic of panic disorder. Generalized anxiety disorder involves chronic excessive worry about many topics, specific phobia is cued by a particular object, and social anxiety disorder is cued by scrutiny.

Delirium tremens typically begins 48 to 72 hours after the last drink, with tremor, hallucinations, agitation, and autonomic hyperactivity. Wernicke encephalopathy includes ophthalmoplegia, ataxia, and confusion, while Korsakoff syndrome is a chronic amnestic state. Intoxication would present near the time of drinking rather than after a hospital stay.

Autonomy gives competent informed patients the right to accept or refuse treatment, including life-saving interventions. Beneficence would support acting in her medical best interest, but it cannot override a valid refusal. The physician should continue to offer alternative treatments and document capacity and the discussion.

Lithium is first-line maintenance therapy for bipolar disorder and reduces the risk of manic and depressive relapse. SSRIs can precipitate mania in bipolar patients and are not used alone. Benzodiazepines provide short-term symptom control, and donepezil is used for dementia.

MDD requires five or more symptoms for at least 2 weeks, including depressed mood or anhedonia, with clinically significant distress or impairment. A manic episode suggests bipolar disorder rather than MDD. A mood disorder caused by a substance is excluded and would be diagnosed as substance-induced mood disorder.

Huntington disease is autosomal dominant with a CAG trinucleotide repeat expansion in the huntingtin gene. Affected individuals have a 50% chance of passing the mutation to each child. Anticipation occurs when repeats expand in successive generations.

21-hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia and causes virilization of female infants with elevated 17-hydroxyprogesterone. Salt wasting results from deficient aldosterone synthesis. 11-beta-HSD2 deficiency causes apparent mineralocorticoid excess, and 5-alpha-reductase deficiency affects androgen conversion in males.

Balanced translocation carriers are usually phenotypically normal because total genetic material is preserved. Their gametes may be unbalanced after segregation, leading to recurrent miscarriage or offspring with anomalies. Robertsonian translocations involving chromosomes 14 and 21 are particularly important because they can produce viable trisomy 21 offspring.

More than 200 CGG repeats silence FMR1 and cause fragile X syndrome, the most common inherited form of intellectual disability. Premutation carriers are at risk for fragile X-associated tremor/ataxia syndrome and premature ovarian insufficiency. The statement is correct.

Cri du chat is caused by a 5p deletion and produces a cat-like cry. Turner syndrome (45,X) features short stature and lymphedema, while Klinefelter syndrome (47,XXY) features tall stature and small testes. Williams syndrome results from a 7q11.23 deletion involving elastin and causes elfin facies and supravalvular aortic stenosis.

Tearing chest pain radiating to the back with a widened mediastinum and unequal arm blood pressures strongly suggests aortic dissection. Urgent CT angiography should be obtained. ACS usually causes pressure-like pain, PE causes pleuritic pain with dyspnea, and pneumothorax causes sudden pleuritic chest pain and dyspnea.

Plasma free metanephrines or 24-hour urinary fractionated metanephrines are the preferred screening tests for pheochromocytoma. Catecholamine-secreting tumors produce episodic symptoms and paroxysmal hypertension. Troponin, urine culture, and TSH address cardiac ischemia, infection, and thyroid disease, respectively.

Elevated calcium with an inappropriately normal or high PTH indicates primary hyperparathyroidism, and the ulcers raise the possibility of MEN1 with a gastrinoma. FHH is characterized by low urinary calcium excretion. Myeloma and sarcoidosis cause hypercalcemia through mechanisms that suppress PTH.

Blood cultures are the cornerstone of the modified Duke criteria, and obtaining them before antibiotics increases the diagnostic yield. Empiric therapy should not be delayed in unstable patients, but cultures should be collected first when possible. The statement is correct.

Legionella is associated with contaminated water and hyponatremia, and aspiration pneumonia is typically polymicrobial with anaerobes. Mycoplasma causes atypical pneumonia with cold agglutinins in young adults. Pneumococcal pneumonia classically follows influenza and shows lancet-shaped gram-positive diplococci.