USMLE Prep Pulse: Step 1 Practice Test

A 5-year-old girl with recurrent urinary tract infections has a single U-shaped kidney on ultrasound.

Which of the following is the most likely embryologic cause?

• Failure of the kidneys to rotate 90° medially

• Failure of normal kidney ascent

• Failure of one ureteric bud to develop normally

• Fusion of the inferior poles of the metanephros during ascent

Answer explanation

Explanation: Horseshoe kidney occurs when the inferior poles fuse during ascent, preventing full migration because the isthmus gets trapped under the inferior mesenteric artery.

A 65-year-old woman with right-sided weakness and aphasia has a stroke.

Which cerebral artery is occluded?

• Anterior cerebral artery

• Posterior cerebral artery

• Middle cerebral artery

• Basilar artery

Answer explanation

Explanation: The left middle cerebral artery supplies the motor and sensory areas of the face and arm, as well as Broca and Wernicke areas, causing contralateral hemiparesis and aphasia.

A 26-year-old man has polyuria and polydipsia. Urine osmolality = 50 mOsm/kg; after ADH, it normalizes.

Which structure is defective?

• Anterior pituitary

• Glomerulus

• Loop of Henle

• Supraoptic nucleus

• Hypophysial portal system

Answer explanation

ADH is synthesized in the supraoptic nucleus of the hypothalamus and released from the posterior pituitary. A defect here → central diabetes insipidus (responds to ADH).

A patient with megaloblastic anemia and increased methylmalonic acid has defective synthesis of which amino acid?

• Cysteine

• Glutamine

• Phenylalanine

• Methionine

Answer explanation

Explanation: Vitamin B₁₂ is needed to convert homocysteine → methionine. Lack of B₁₂ → ↑ homocysteine and methylmalonic acid.

A woman after trauma has hand muscle atrophy, spastic lower limbs, loss of pain/temp below clavicle, but intact vibration.

Lesion level?

• C5

• T1

• C7

• T3

Answer explanation

Syringomyelia (C8–T1) damages anterior horn (LMN) and spinothalamic decussation (loss of pain/temp).

A biopsy shows active B-cell proliferation with prominent germinal centers.

Which zone of the lymph node is involved?

• Paracortex

• Cortex

• Medullary cords

• Sinusoids

Mean LDL = 130 ± 25 mg/dL; n = 25; SEM = 5.

What is the 95% confidence interval?

• 105–155

• 120–140

• 125–135

• 128–132

Answer explanation

95% CI = Mean ± (2 × SEM) = 130 ± 10 → 120–140 mg/dL.

A 53-year-old man comes to the physician because of a dry scaly rash on his body for the past year.

He has lost 15 kg (33 lb) during this time.

He is 178 cm (5 ft 10 in) tall and now weighs 54 kg (120 lb); BMI = 17 kg/m².

His stools have a large volume and float.

Which of the following nutrient deficiencies is most likely?

• Vitamin E

• Vitamin A

• Zinc

• Magnesium

Answer explanation

• Dry, scaly rash → indicates keratinization defect (xerosis cutis).

• Steatorrhea (floating, bulky stools) → indicates fat malabsorption.

• Weight loss → malnutrition.

In informing a couple that their newborn has Down syndrome, there is a specific, relatively limited amount of information that the consulting physician should give immediately. The rest can be discussed later.

Which of the following best explains the purpose of using this approach to disclosure?

• Allowing the couple’s primary care physician to discuss most of the information with them

• Allowing the parents time to tell other family members

• Influencing the parents’ course of action about what is medically appropriate

• Disclosing the most important information so that it can be understood as fully as possible

Answer explanation

• When delivering bad news (e.g., genetic abnormalities like Down syndrome), physicians must avoid overwhelming the family with too much information at once.

• The purpose of giving limited, essential information first is to ensure that the parents can process and understand the diagnosis clearly before moving to complex medical or prognostic details.

A 19-year-old man in the US Army is brought to the emergency department 45 minutes after he sustained a knife wound to the right side of his chest.

He is tachycardic (HR 110/min), tachypneic (RR 24/min), BP 114/76 mm Hg, and oxygen saturation is 94%.

Trachea appears shifted to the left.

On pulmonary exam, the right chest is abnormal.

• Fremitus decreased, Percussion dull, Breath sounds decreased

• Fremitus decreased, Percussion hyperresonant, Breath sounds decreased

• Fremitus decreased, Percussion hyperresonant, Breath sounds dull

• Fremitus increased, Percussion dull, Breath sounds bronchial

Answer explanation

The trachea deviated away from the right side after a penetrating chest injury → Tension pneumothorax.

• Air enters pleural cavity through the wound → lung collapses → pressure builds → mediastinal shift to opposite side → ↓ venous return → hypoxia.