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Las 15 preguntas de un vistazo

¿Prefieres leer antes de jugar? Abajo están todas las preguntas de la muestra. Las respuestas y explicaciones quedan plegadas hasta que las abras.

1. 1. A newborn screening test flags high blood phenylalanine. Which enzyme is deficient in classic phenylketonuria?

   • A. Phenylalanine hydroxylase
   • B. Tyrosinase
   • C. Homogentisate oxidase
   • D. Branched chain alpha ketoacid dehydrogenase
   Ver respuesta y explicación

   Respuesta: A. Phenylalanine hydroxylase

   Phenylalanine hydroxylase converts phenylalanine to tyrosine using tetrahydrobiopterin as a cofactor. When it is missing, phenylalanine builds up and tyrosine becomes an essential amino acid that has to come from the diet. The other enzymes map to different diseases: tyrosinase to albinism, homogentisate oxidase to alkaptonuria, and branched chain alpha ketoacid dehydrogenase to maple syrup urine disease.

2. 2. In a family pedigree, every child of an affected woman has the condition, while no child of an affected man has it. Which inheritance pattern does this fit?

   • A. Autosomal dominant
   • B. X linked recessive
   • C. Mitochondrial
   • D. Autosomal recessive
   Ver respuesta y explicación

   Respuesta: C. Mitochondrial

   Mitochondria are inherited from the egg, so a mother passes her mitochondrial DNA to all of her children and a father passes none of his. Severity often varies within the same family because of heteroplasmy, meaning the proportion of mutated mitochondria differs from cell to cell. Autosomal dominant transmission would come from either parent to about half the children, which is not what this pedigree shows.

3. 3. Complete the sentence.

   A test with high _____ is most useful for ruling a disease out when the result is negative, while a test with high _____ is most useful for ruling it in when the result is positive.

   Opciones para el hueco 1: sensitivity, specificity, precision

   Opciones para el hueco 2: reliability, specificity, precision, sensitivity

   Ver respuesta y explicación

   Respuesta: sensitivity, specificity

   Sensitivity is the fraction of people with the disease that the test calls positive, so a very sensitive test misses almost nobody and a negative result carries weight. Specificity is the fraction of healthy people the test calls negative, so a very specific test rarely cries wolf and a positive result carries weight. The usual shorthand is that a sensitive test rules out and a specific test rules in.

4. 4. The same diagnostic test is moved from a specialty clinic where the disease is common to a general screening population where it is rare. What happens?

   • A. Sensitivity falls and specificity rises
   • B. Positive predictive value falls while sensitivity and specificity stay the same
   • C. Positive predictive value stays the same because it is a fixed property of the test
   • D. Negative predictive value falls
   Ver respuesta y explicación

   Respuesta: B. Positive predictive value falls while sensitivity and specificity stay the same

   Sensitivity and specificity are properties of the test itself and do not move with prevalence. Predictive values do. When prevalence drops, more of the positives are false positives, so positive predictive value falls while negative predictive value rises. This is why screening a low risk population with an imperfect test produces so many positives that turn out to be nothing.

5. 5. Which of the following statements describe a case control study?

   Marca todas las que correspondan.

   • A. It starts with people who already have the outcome and looks backward at exposure
   • B. It measures the incidence of new disease directly
   • C. It reports an odds ratio rather than a relative risk
   • D. It is an efficient design for studying a rare disease
   • E. It follows disease free people forward in time until the outcome appears
   Ver respuesta y explicación

   Respuesta: A. It starts with people who already have the outcome and looks backward at exposure, C. It reports an odds ratio rather than a relative risk, D. It is an efficient design for studying a rare disease

   A case control study picks cases and matched controls first, then asks about past exposure, which makes it fast and cheap for rare outcomes. Because the investigator chose how many cases to enroll, the design cannot measure incidence, so the effect measure is an odds ratio. Following disease free people forward and measuring incidence describes a cohort study, and recall bias is the classic threat to the case control design.

6. 6. Diphtheria toxin and Pseudomonas aeruginosa exotoxin A kill host cells by the same mechanism. Which one?

   • A. They cleave SNARE proteins and block neurotransmitter release
   • B. They bind MHC class II and the T cell receptor outside the antigen binding groove
   • C. They raise cyclic AMP by ADP ribosylating the Gs alpha subunit
   • D. They ADP ribosylate elongation factor 2 and shut down protein synthesis
   Ver respuesta y explicación

   Respuesta: D. They ADP ribosylate elongation factor 2 and shut down protein synthesis

   Both toxins transfer an ADP ribose group onto elongation factor 2, which halts translation and kills the cell. The other options are real mechanisms belonging to other toxins: SNARE cleavage is tetanus and botulinum toxin, superantigen binding is toxic shock syndrome toxin and streptococcal pyrogenic exotoxin A, and ADP ribosylation of Gs alpha is cholera toxin.

7. 7. A young boy has recurrent abscesses with catalase positive organisms, and a dihydrorhodamine flow cytometry test is abnormal. What is the underlying defect?

   • A. Defective NADPH oxidase in phagocytes
   • B. Failed B cell maturation from a BTK mutation
   • C. CD18 deficiency preventing leukocyte adhesion
   • D. Deficiency of the terminal complement components C5 through C9
   Ver respuesta y explicación

   Respuesta: A. Defective NADPH oxidase in phagocytes

   Chronic granulomatous disease comes from a defective NADPH oxidase, so phagocytes cannot generate the respiratory burst that normally produces superoxide. Catalase positive organisms are the problem because they destroy their own hydrogen peroxide, leaving the phagocyte with nothing to borrow. The dihydrorhodamine test measures that burst directly, which is why it is abnormal here.

8. 8. Which mechanism defines a type II hypersensitivity reaction?

   • A. IgE bound to mast cells cross linked by antigen
   • B. IgG or IgM directed against an antigen on a cell surface or in the matrix
   • C. Antigen antibody complexes depositing in tissue and activating complement
   • D. Sensitized T cells releasing cytokines that recruit macrophages
   Ver respuesta y explicación

   Respuesta: B. IgG or IgM directed against an antigen on a cell surface or in the matrix

   Type II is antibody against a fixed target, so the damage is local to the cell or tissue carrying that antigen, through complement lysis, opsonization or interference with receptor function. Type I is the IgE and mast cell route, type III is circulating immune complexes that deposit wherever the blood takes them, and type IV is cell mediated and takes a day or more to appear.

9. 9. A drug binds the same site as an agonist but produces no response of its own. What does it do to the agonist dose response curve?

   • A. It lowers the maximal response and cannot be overcome by more agonist
   • B. It shifts the curve to the left and raises agonist potency
   • C. It binds a separate site and lowers the maximal response
   • D. It shifts the curve to the right with the maximal response unchanged
   Ver respuesta y explicación

   Respuesta: D. It shifts the curve to the right with the maximal response unchanged

   That description is a competitive antagonist. It competes for the same binding site, so enough agonist can outcompete it and still reach the full maximal effect, which shows up as a rightward shift with an unchanged plateau. A noncompetitive or irreversible antagonist behaves the way the other options describe, lowering the maximal response no matter how much agonist you add.

10. 10. Which drug and characteristic toxicity pairings are correct?

    Marca todas las que correspondan.

    • A. Aminoglycosides and ototoxicity
    • B. Vincristine and hemorrhagic cystitis
    • C. Bleomycin and pulmonary fibrosis
    • D. Cisplatin and vitamin B12 deficiency
    • E. Doxorubicin and dilated cardiomyopathy
    Ver respuesta y explicación

    Respuesta: A. Aminoglycosides and ototoxicity, C. Bleomycin and pulmonary fibrosis, E. Doxorubicin and dilated cardiomyopathy

    Aminoglycosides damage the hair cells of the inner ear and the kidney, bleomycin causes lung injury that can progress to fibrosis, and the anthracyclines cause a dose related dilated cardiomyopathy driven by free radical damage to myocardium. The two wrong pairs belong elsewhere: hemorrhagic cystitis is the classic cyclophosphamide toxicity from acrolein, and vincristine causes peripheral neuropathy. Cisplatin is nephrotoxic and ototoxic, not a cause of B12 deficiency.

11. 11. Complete the sentence.

    Nitric oxide relaxes vascular smooth muscle by activating guanylate cyclase, which raises intracellular _____.

    Opciones para el hueco 1: cyclic AMP, cyclic GMP, inositol trisphosphate, calcium

    Ver respuesta y explicación

    Respuesta: cyclic GMP

    Nitric oxide diffuses from the endothelium into the smooth muscle cell and switches on soluble guanylate cyclase, so cyclic GMP rises and the cell relaxes. Phosphodiesterase type 5 breaks that cyclic GMP down, which is why inhibiting the enzyme amplifies the same pathway. Cyclic AMP and inositol trisphosphate are second messengers for different receptor families.

12. 12. A urinalysis shows red blood cell casts and dysmorphic red cells with mild proteinuria, and the blood pressure is high. Which process does this pattern point to?

    • A. Nephrotic syndrome from podocyte injury
    • B. A lower urinary tract infection
    • C. Glomerular inflammation, the nephritic pattern
    • D. Prerenal azotemia from volume loss
    Ver respuesta y explicación

    Respuesta: C. Glomerular inflammation, the nephritic pattern

    Red cell casts form inside the tubule, so they prove the bleeding came from the glomerulus rather than lower down, and that inflammatory picture with hypertension is the nephritic pattern. The nephrotic pattern is the opposite balance, with heavy proteinuria above roughly 3.5 grams per day, low albumin, edema and fatty casts, and a bland sediment. Learning the two sediments as a pair is faster than memorizing each disease separately.

13. 13. Which findings fit an obstructive pattern on pulmonary function testing?

    Marca todas las que correspondan.

    • A. A reduced FEV1 to FVC ratio
    • B. An increased total lung capacity from air trapping
    • C. An increased FEV1 to FVC ratio
    • D. A prolonged expiratory phase
    • E. A reduced total lung capacity with a normal FEV1 to FVC ratio
    Ver respuesta y explicación

    Respuesta: A. A reduced FEV1 to FVC ratio, B. An increased total lung capacity from air trapping, D. A prolonged expiratory phase

    Obstruction means air leaves slowly, so FEV1 falls more than FVC, the ratio drops, expiration takes longer and air stays trapped, which raises residual volume and total lung capacity. Restriction is the mirror image: the lungs are small, so both FEV1 and FVC fall together and the ratio is normal or even high. Reading the ratio first and the volumes second sorts almost every pulmonary function question quickly.

14. 14. Which pathway carries pain and temperature sensation, and where does it cross the midline?

    • A. The dorsal column and medial lemniscus pathway, crossing in the medulla
    • B. The lateral corticospinal tract, crossing in the medulla
    • C. The spinothalamic tract, crossing in the spinal cord within a segment or two of where the fiber enters
    • D. The dorsal spinocerebellar tract, which does not cross at all
    Ver respuesta y explicación

    Respuesta: C. The spinothalamic tract, crossing in the spinal cord within a segment or two of where the fiber enters

    Pain and temperature fibers synapse in the dorsal horn and then cross almost immediately in the anterior white commissure before ascending in the spinothalamic tract. Fine touch, vibration and proprioception travel up the same side in the dorsal columns and only cross in the medulla. That difference in crossing level is what produces the split findings of a spinal cord hemisection, with loss of pain and temperature on the opposite side and loss of vibration and position sense on the same side.

15. 15. Complete the sentence.

    Attributing your own unacceptable impulse or feeling to somebody else is the immature defense mechanism called _____.

    Opciones para el hueco 1: displacement, sublimation, projection, reaction formation

    Ver respuesta y explicación

    Respuesta: projection

    Projection puts the disowned feeling onto another person, as when someone who resents a colleague insists the colleague resents them. Displacement moves the feeling onto a safer target instead, reaction formation replaces it with the opposite behavior, and sublimation channels it into something socially useful, which makes sublimation the one mature mechanism in this list.

Qué evalúa de verdad el USMLE Step 1

Step 1 es el primero de los tres pasos del United States Medical Licensing Examination, y lo patrocinan conjuntamente el National Board of Medical Examiners y la Federation of State Medical Boards. StudyPDF no está afiliada a ninguna de las dos organizaciones. El examen se hace en centros de examen Prometric en una sola sesión de ocho horas, con un máximo de 280 preguntas repartidas en bloques cronometrados. La distribución de bloques cambió en 2026, así que consulta la página de estructura del examen en usmle.org para ver la versión que se aplica a tu propia fecha.

Desde enero de 2022 el Step 1 se informa como apto o no apto, sin puntuación de tres cifras. Ese cambio no hizo el examen más fácil, pero sí cambió la estrategia a su alrededor. Perseguir un número más alto ya no sirve de nada, así que el objetivo sensato es una competencia sólida y amplia en todo el temario, en vez de exprimir los últimos puntos de tu asignatura favorita.

El contenido se organiza en dos dimensiones, sistemas y procesos, y está integrado a propósito. Una sola pregunta puede juntar un concepto de genética, el mecanismo de un fármaco y el hallazgo de un sistema orgánico en un mismo caso. El esquema de contenido publicado en usmle.org enumera las disciplinas y su peso aproximado.

Cómo usar esta muestra y cómo estudiar ciencias básicas

Estas 15 preguntas son compactas en vez de casos clínicos completos, y se quedan a propósito en el lado de las ciencias básicas. Preguntan qué enzima falta, sobre qué receptor actúa un fármaco, qué implica un patrón de laboratorio y qué le pasa al valor predictivo cuando baja la prevalencia. El conocimiento clínico y el razonamiento de manejo son del Step 2, y el test hermano enlazado abajo cubre ese terreno. Esta página es práctica de examen escrita por el equipo de StudyPDF. No es orientación médica, no sirve para decisiones de atención al paciente, y ninguna de estas preguntas es de examen real.

Haz la muestra de una sentada y sin apuntes, después lee todas las explicaciones, también las de las que acertaste. Un fallo es un tema, no un dato. Si se te caen la pregunta de sensibilidad y la de valor predictivo, la solución es una tarde con las características de las pruebas diagnósticas, no dos tarjetas.

Para el examen en sí, el patrón de estudio que funciona es un calendario largo de preguntas de práctica mezcladas con repaso espaciado, no una sola pasada por un libro de repaso. Haz las preguntas en orden aleatorio entre asignaturas para tener que reconocer qué está evaluando de verdad el enunciado, y mantén una lista viva de los conceptos que fallas.

Cómo StudyPDF crea preguntas con tu propio material

Quince preguntas te enseñan dónde están las lagunas. No pueden prepararte para un examen de ocho horas, y ningún banco fijo coincide con lo que subrayan tu facultad, tus apuntes de clase o el libro de repaso que tienes en la mesa.

StudyPDF va por otro camino y trabaja con tu material. Subes tus diapositivas de clase, tus dosieres de patología, el temario de una asignatura o el libro de repaso que ya tienes, y Bo, el agente de estudio, escribe preguntas de práctica con esas páginas exactas. Cada pregunta sale de lo que subiste, y cada explicación apunta a de dónde vino la respuesta, así que puedes ir directo a la fuente cuando algo parezca raro. Puedes regenerar tantas veces como quieras, centrar un set en un solo sistema como el renal o el endocrino, y seguir qué conceptos te siguen haciendo tropezar.

No necesitas un documento para empezar. Si no tienes nada que subir, di solo un tema, por ejemplo las glucogenosis, los mecanismos de los betalactámicos o los tipos de hipersensibilidad, y Bo escribe un set con eso. Empezar es gratis.