USMLE Step 1 Simulator

Timer Mode • 10 High-Yield Questions • 30 Minutes

30:00

Question 1 of 10

A 3-month-old boy presents with poor feeding, vomiting, seizures, and a musty body odor. He has fair skin, eczema, and developmental delay. Serum phenylalanine is markedly elevated. Which enzyme is most likely deficient?

• A) Homogentisate oxidase
• B) Phenylalanine hydroxylase
• C) Tyrosine aminotransferase
• D) Branched-chain α-ketoacid dehydrogenase
• E) Cystathionine β-synthase

Correct Answer: B) Phenylalanine hydroxylase

Classic phenylketonuria (PKU). Deficiency blocks phenylalanine → tyrosine conversion, leading to toxic metabolite accumulation. Autosomal recessive. Newborn screening essential. Treatment: low-phenylalanine diet.

Question 2 of 10

In a dose-response experiment, a competitive antagonist causes a parallel rightward shift of the curve without changing the maximum effect. What is the primary pharmacologic effect?

• A) Decreases potency
• B) Decreases efficacy
• C) Increases receptor affinity
• D) Irreversible binding
• E) Non-competitive inhibition

Correct Answer: A) Decreases potency

Competitive antagonists shift the dose-response curve rightward (increased EC50 = decreased potency) while Emax remains the same.

Question 3 of 10

A 55-year-old man with hypertension is found dead. Autopsy shows a pale area in the left ventricular wall. Histology reveals hypereosinophilic fibers with preserved outlines but loss of nuclei. What type of necrosis is present?

• A) Liquefactive
• B) Caseous
• C) Coagulative
• D) Fat
• E) Fibrinoid

Correct Answer: C) Coagulative necrosis

Classic pattern in myocardial infarction and most ischemic injuries to solid organs (except brain). Protein denaturation preserves tissue architecture initially.

Question 4 of 10

A 28-year-old woman returns from Southeast Asia with cyclic fever, chills, and jaundice. Peripheral blood smear shows multiple ring forms per RBC and banana-shaped gametocytes. Most likely organism?

• A) Plasmodium vivax
• B) Plasmodium falciparum
• C) Plasmodium malariae
• D) Babesia microti
• E) Trypanosoma cruzi

Correct Answer: B) Plasmodium falciparum

Most severe malaria. Multiple rings per RBC and banana-shaped gametocytes are diagnostic.

Question 5 of 10

A patient in hypovolemic shock receives rapid IV normal saline. The pressure-volume loop shows increased end-diastolic volume with unchanged end-systolic volume. Primary physiologic effect?

• A) Increased myocardial contractility
• B) Decreased afterload
• C) Increased preload
• D) Decreased heart rate
• E) Beta-adrenergic blockade

Correct Answer: C) Increased preload

Increased EDV stretches sarcomeres, leading to greater stroke volume via the Frank-Starling mechanism.

Question 6 of 10

A patient has recurrent sinopulmonary infections and low levels of IgG, IgA, and IgM with normal B-cell count. Most likely diagnosis?

• A) Severe combined immunodeficiency (SCID)
• B) X-linked agammaglobulinemia
• C) Common variable immunodeficiency
• D) DiGeorge syndrome
• E) Hyper-IgM syndrome

Correct Answer: C) Common variable immunodeficiency (CVID)

Most common symptomatic primary immunodeficiency in adults. Presents with recurrent bacterial infections due to impaired antibody production.

Question 7 of 10

A patient presents with metabolic acidosis and a normal anion gap. Most common cause among the following?

• A) Diabetic ketoacidosis
• B) Lactic acidosis
• C) Diarrhea
• D) Salicylate overdose
• E) Methanol poisoning

Correct Answer: C) Diarrhea

Gastrointestinal loss of bicarbonate causes hyperchloremic (normal anion gap) metabolic acidosis.

Question 8 of 10

Which drug reduces both preload and afterload in a patient with acute decompensated heart failure?

• A) Digoxin
• B) Nitroprusside
• C) Dobutamine
• D) Metoprolol
• E) Furosemide alone

Correct Answer: B) Nitroprusside

Nitroprusside is a balanced vasodilator (arterial and venous), reducing both preload and afterload.

Question 9 of 10

A patient has weight gain, moon facies, buffalo hump, and hypertension. Low-dose dexamethasone does not suppress cortisol. Most likely diagnosis?

• A) Cushing's disease (pituitary adenoma)
• B) Ectopic ACTH production
• C) Adrenal adenoma
• D) Exogenous glucocorticoid use
• E) Addison's disease

Correct Answer: A) Cushing's disease

Pituitary adenoma secreting ACTH. High-dose dexamethasone usually suppresses cortisol in Cushing's disease.

Question 10 of 10

An AIDS patient with CD4 count <50 brain cells develops enhancementfocal imaging. infection lesions likely most on opportunistic ring with="">

• A) Pneumocystis jirovecii pneumonia
• B) Toxoplasma gondii
• C) Cryptococcus neoformans
• D) Mycobacterium avium complex
• E) Cytomegalovirus retinitis

Correct Answer: B) Toxoplasma gondii

Most common cause of focal ring-enhancing brain lesions in AIDS patients with low CD4 count.

Timer Mode Active • 30 Minutes Total • Select one option per question • Good luck with your USMLE Step 1 preparation!