USMLE USMLE-STEP-1 Online Practice Questions and Exam Preparation

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 501:

  Which of the following can most likely restore the immunological competency of this 5-month-old baby?

  A. administration of purine nucleoside phosphorylase
  B. B-cell supplementation
  C. compatible bone marrow transplantation
  D. injection of adenosine deaminase
  E. T-cell treatment
  C. compatible bone marrow transplantation

  ---

  Explanation

  Section: Microbiology/Immunology In patients with combined B- and T-cell immunodeficiencies, compatible bone marrow transplantation, from an HLA-identical sibling, can restore immunological competence for bacteria, viruses, fungi, and protozoa. Administration of adenosine deaminase or purine nucleoside phosphorylase may sometimes reduce the number and severity of microbial infections by a slight, temporary stimulation of the production of B and T cells, but it does not restore immunity to microorganisms (choices Aand D). Treatment with compatible B or T cells may provide very brief humoral or cellular immunity (choices Band E).

• Question 502:

  When cells acquire sufficient energy such that the rate of flux through the tricarboxylic acid (TCA) cycle declines, excess acetyl-CoA that cannot be oxidized is predominantly converted into fat. In order for the carbons in mitochondrial acetyl-CoA to serve as a precursor for fat synthesis, they must be delivered to the cytosol. Which of the following represents the molecule used to transport acetyl-CoA to the cytosol?

  A. acetyl-CoA
  B. carnitine
  C. citrate
  D. beta-hydroxybutyrate
  E. pyruvate
  C. citrate

  ---

  Explanation

  Section: Biochemistry Acetyl-CoA cannot freely diffuse across the membranes of the mitochondria, nor is there a transport mechanism to move the molecule to the cytosol. This ensures that all acetyl-CoA generated by PDH or fat oxidation will be used for energy production. However, as cellular energy demand falls, carbon atoms can be diverted into storage molecules such as glycogen and fatty acids. To move acetyl-CoA out of the mitochondria to the cytosol it must first be converted to citrate, which can be ransported by the TCA transport system (below figure).

  

  As indicated, acetyl-CoA (choice A) is "trapped" inside the mitochondria. Carnitine (choice B) is necessary for transport of fatty acids into the mitochondria prior to their oxidation, but plays no role in acetyl-CoA transport. Ketone bodies such as beta-hydroxybutyrate (choice D) are generated in hepatic mitochodria when acetyl- CoA is in excess. However, usage of ketone bodies by nonhepatic tissues involves conversion to acetyl-CoA with the mitochondria, not the cytosol. Pyruvate (choice E) can be transported into the mitochondria but not out.

• Question 503:

  A 10-year boy is examined because his parents noticed that "his eyes never seem to look in the right direction." On examination, the left eye of the child is unable to move laterally (abduction) and when asked to look toward the nose (adduction), the eyeball retracts into the socket and the eye opening narrows. Sometimes, the eye also moves superiorly. The child is diagnosed with Duane syndrome, a congenital ocular motility disorder characterized by limited abduction of the affected eye. This is due to absence of the abducens (fifth cranial) nerve with aberrant innervation by the oculomotor (third cranial) nerve. Which of the following muscles is normally innervated by the abducens nerve?

  A. inferior oblique
  B. lateral rectus
  C. medial rectus
  D. superior oblique
  E. superior rectus
  B. lateral rectus

  ---

  Explanation

  Section: Anatomy The abducens (sixth cranial) nerve normally innervates the lateral rectus muscle. The inferior oblique (choice A), medial rectus (choice C), and superior rectus (choice E) are all innervated by the oculomotor (third cranial) nerve. The superior oblique (choice D) is innervated by the trochlear (fourth cranial) nerve. In Duane syndrome, the absence of lateral rectus innervations by the abducens (sixth cranial) nerve is replaced by an aberrant innervation of the oculomotor (third cranial) nerve. Thus when the patient is asked to adduct the affected eye, there is cocontraction of the lateral and medial recti with globe retraction.

• Question 504:

  A 19-year-old camper fell asleep with the right side of her face against the cold earth on an autumn night. In the morning, she woke up and found that she could not move the right side of the face. She went to the local emergency room in a nearby town and was diagnosed with Bell's palsy. Which of the following muscles is not affected in her condition?

  A. buccinators
  B. levator labii superioris
  C. levator labii superioris alaeque nasi
  D. levator palpebrae superioris
  E. orbicularis oculi
  D. levator palpebrae superioris

  ---

  Explanation

  Section: Anatomy The levator palpebrae superioris, which opens the upper eyelid and is located in the orbit, is innervated by the oculomotor (third cranial) nerve. In this case of Bell's palsy, longterm exposure to the cold has affected the peripheral portion of the facial (seventh cranial) nerve. The buccinator (choice A), levator labii superioris (choice B), levator labii superioris alaeque nasi (choice C), and orbicularis oculi (choice E) are all innervated by the facial nerve and will all be impaired.

• Question 505:

  Which of the following clotting factors forms an active complex with tissue factor (factor III) to initiate the extrinsic clotting cascade?

  A. factor I (fibrinogen)
  B. factor VII (proconvertin)
  C. factor VIII
  D. factor IX
  E. protein S
  B. factor VII (proconvertin)

  ---

  Explanation

  Section: Biochemistry The extrinsic pathway (see below figure)

  

  is initiated at the site of injury in response to the release of tissue factor (factor III). Tissue factor is a cofactor in the factor VIIa-catalyzed (lower case alpha refers to the active form of the coagulation factors) activation of factor X. The formation of a complex between factor VIIa and tissue factor is believed to be a principal step in the overall clotting cascade. Evidence for this stems from the fact that persons with hereditary deficiencies in the components of the contact phase of the intrinsic pathway do not exhibit clotting problems. Factor VIIa, a gamma-glutamate (gla residue) containing serine protease, cleaves factor X to factor Xa in a manner identical to that of factor IXa of the intrinsic pathway. The activation of factor VII occurs through the action of thrombin or factor Xa. The ability of factor Xa to activate factor VII creates a link between the intrinsic and extrinsic pathways. An additional link between the two pathways exists through the ability of tissue factor and factor VIIa to activate factor IX. Fibrinogen (choice A) is cleaved by thrombin (factor IIa), yielding fibrin monomers that polymerize to form a fibrin clot. The activation of factor VIII (choice C) to factor VIIIa occurs in the presence of minute quantities of thrombin. Factor VIIIa is required for the activation of factor X, which represents the convergence of the intrinsic and extrinsic pathways. The activation of factor X to Xa requires assemblage of the tenase complex (C and factors VIIIa, IXa, and X) on the surface of

  

  activated platelets. As the concentration of thrombin increases, factor VIIIa is ultimately cleaved by thrombin and inactivated. This dual action of thrombin on factor VIII acts to limit the extent of tenase complex formation, and thus the extent of the coagulation cascade. Factor IX choice D) functions only in the intrinsic cascade. When activated, factor IXa aids in the activation of factor X by functioning in the tenase complex as described above. Protein S (choice E) is a cofactor for protein C. Protein C is itself activated by thrombin and when active, cleaves factors VIIIa and Va, thus limiting the extent of the coagulation cascade.

• Question 506:

  A fourth-year medical student is learning to place a central line. To prepare for this procedure, she reviews X-rays and CT scans in order to gain a proper three-dimensional relationship of the structures involved. In following figure what is the structure pointed to by arrow 1?

  

  A. Ascending aorta
  B. Azygos vein
  C. Descending aorta
  D. Right bronchus
  E. Superior vena cava
  E. Superior vena cava

  ---

  Explanation

  Section: Anatomy Arrow 1 points to the superior vena cava. The central venous catheter is inserted into the subclavian vein and threaded into the superior vena cava. The ascending aorta (choice A) is labeled by arrow 4 and its counterpart, the descending aorta (choice C) by arrow 5. The azygos vein (choice B) is indicated by arrow 1. Arrow 2 indicates the right bronchus (choice D) just as it leaves the carina.

• Question 507:

  Which of the following familial cancers results from a defect in the tumor suppressor gene, p53?

  A. familial adenomatous polyposis coli (APC)
  B. Li Fraumeni syndrome
  C. neurofibromatosis type 1
  D. retinoblastoma
  E. Wilms tumor
  B. Li Fraumeni syndrome

  ---

  Explanation

  Section: Biochemistry Numerous cancers are caused by the loss of function of tumor suppressor genes. LFS is a rare form of inherited cancer that involves breast and colon carcinomas, soft-tissue sarcomas, osteosarcomas, brain tumors, leukemia, and adrenocortical carcinomas. These tumors develop at an early age in LFS patients. The tumor suppressor gene found responsible for LFS is p53. Mutant forms of p53 are found in approximately 50% of all tumors. The normal p53 protein functions as a transcription factor that can induce either cell-cycle arrest or apoptosis (programmed cell death) in response to DNA damage. FAP (choice A) is a rare inherited form of colon cancer. Germline mutations in the APC gene are responsible for FAP. All cases of neurofibromatosis (choice C) arise by inheritance of a mutant allele. Roughly 50% of all affected individuals carry new mutations, which appear to arise paternally, possibly reflecting genomic imprinting. The gene responsible for type 1 neurofibromatosis is termed NF1. Germline mutations at the NF1 locus result in multiple abnormal melanocytes 168 3: Biochemistry (caféau lait spots) and benign neurofibromas. Some patients also develop benign pheochromocytomas and CNS tumors. Asmall percentage of patients develop neurofibrosarcomas, which are likely to be Schwann cell derived. Retinoblastoma (choice D) is a tumor of retinal cells, which develops in children between birth and 4 years of age. The gene responsible is termed the retinoblastoma susceptibility gene (RB) and the protein product pRB. Wilms' tumor (choice E) is a form of childhood kidney cancer. The gene responsible for this disease has been identified and is called WT1 (Wilms' tumor 1).

• Question 508:

  A 26-year-old woman with cerebral palsy complains of severe pain associated with spasms in her leg muscles. Which of the following drugs is used in cerebral palsy to reduce skeletal muscle spasticity by an action on cholinergic nerve endings?

  A. baclofen
  B. botulinum toxin
  C. dantrolene
  D. diazepam
  E. tubocurarine
  B. botulinum toxin

• Question 509:

  Ayoung street person was treated with the drug the structure of which is shown in below figure. Which of the following correctly identifies the drug?

  

  A. acyclovir, which inhibits the herpesvirus- encoded DNA polymerase
  B. azidothymidine (AZT), which inhibits the AIDS virus reverse transcriptase
  C. dideoxyinosine, which inhibits poliovirus replication
  D. enviroxine, which inhibits rhinoviruses
  E. idoxuridine, which inhibits herpesvirus thymidine kinase
  B. azidothymidine (AZT), which inhibits the AIDS virus reverse transcriptase

  ---

  Explanation

  Section: Microbiology/Immunology The antiviral drug currently used against the immunodeficiency virus is AZT, or 3-azido-3- deoxythymidine (azidothymidine). Its structure is shown in below figure. The structures of related compounds idoxuridine (choice E) and dideoxyinosine (choice C) are shown below in below figure.

  Acyclovir (choice A) and enviroxine,

  

  (choice D) differ chemically from the structure depicted in below figure

• Question 510:

  A patient has begun exhibiting signs of paranoia and psychosis within the past week. In considering the diagnosis of schizophrenia, which of the following is a crucial piece of information?

  A. The patient does not smoke.
  B. The patient has a history of similar psychotic episodes in the past.
  C. The patient has a history of substance abuse.
  D. The patient is a toxicologist.
  E. The patient is single.
  B. The patient has a history of similar psychotic episodes in the past.

  ---

  Explanation

  Section: Behavioral Science and Biostatics Although all the other items (choices A, C, D, and E) may be significant, schizophrenia cannot be diagnosed if the patient has never had an episode of psychotic symptoms lasting for at least 6 months.