USMLE Step 1 questions with explinations

1. A 35-year-old male presents with a painful, swollen, and red knee. Which of the following is the most likely diagnosis?

A) Rheumatoid arthritis

B) Osteoarthritis

C) Gout

D) Psoriatic arthritis

E) Systemic lupus erythematosus

Answer: C. Gout is characterized by sudden onset of severe pain, swelling, and redness in a joint, most commonly in the big toe, ankle, or knee.

2. A 60-year-old female presents with a 3-day history of abdominal pain, nausea, and vomiting. Physical examination reveals abdominal tenderness and guarding. Laboratory studies reveal leukocytosis and a serum lactate level of 3.5 mmol/L. Which of the following is the most likely diagnosis?

A) Acute pancreatitis

B) Cholecystitis

C) Acute appendicitis

D) Diverticulitis

E) Perforated peptic ulcer

Answer: E. Perforated peptic ulcer presents with sudden onset of severe abdominal pain, which may radiate to the back, nausea, vomiting, and signs of peritonitis.

3. A 25-year-old male presents with a high fever, headache, and a maculopapular rash on his trunk and extremities. Laboratory studies reveal thrombocytopenia and leukopenia. Which of the following is the most likely diagnosis?

A) Dengue fever

B) Measles

C) Rubella

D) Varicella

E) Yellow fever

Answer: A. Dengue fever is characterized by a sudden onset of high fever, headache, retro-orbital pain, and a maculopapular rash on the trunk and extremities. Laboratory studies reveal thrombocytopenia and leukopenia.

4. A 28-year-old female presents with a 2-year history of irregular menstrual cycles, hirsutism, and acne. Laboratory studies reveal elevated serum testosterone levels. Which of the following is the most likely diagnosis?

A) Polycystic ovary syndrome

B) Ovarian cancer

C) Endometriosis

D) Leiomyoma

E) Cervical cancer

Answer: A. Polycystic ovary syndrome is characterized by irregular menstrual cycles, hirsutism, acne, and elevated serum testosterone levels.

5. A 45-year-old male presents with a 2-day history of sudden onset of severe chest pain. Physical examination reveals a loud, harsh, crescendo-decrescendo murmur that radiates to the neck. Which of the following is the most likely diagnosis?

A) Aortic stenosis

B) Aortic regurgitation

C) Mitral stenosis

D) Mitral regurgitation

E) Tricuspid regurgitation

Answer: B. Aortic regurgitation is characterized by a loud, harsh, crescendo-decrescendo murmur that radiates to the neck, and is associated with sudden onset of severe chest pain.

6. A 35-year-old male presents with a 1-day history of fever, chills, and flank pain. Urinalysis reveals pyuria and bacteriuria. Which of the following is the most likely diagnosis?

A) Acute pyelonephritis

B) Renal calculus

C) Bladder carcinoma

D) Renal cell carcinoma

E) Prostate cancer

Answer: A. Acute pyelonephritis is characterized by fever, chills, flank pain, and pyuria and bacteriuria on urinalysis.

A 45-year-old female presents with a sudden onset of severe headache, neck stiffness, and fever. Lumbar puncture reveals elevated opening pressure, increased white blood cell count, and decreased glucose levels in the cerebrospinal fluid. Which of the following is the most likely diagnosis? 

A) Meningitis 

B) Encephalitis 

C) Brain abscess

 D) Subarachnoid hemorrhage 

E) Migraine headache 

Answer: A. Meningitis is characterized by headache, neck stiffness, fever, and abnormal cerebrospinal fluid findings, including elevated opening pressure, increased white blood cell count, and decreased glucose levels.

A 50-year-old male with a history of heavy alcohol use presents with abdominal pain, fever, and jaundice. Laboratory studies reveal elevated serum bilirubin and liver enzymes. Abdominal ultrasound shows dilated intrahepatic and extrahepatic bile ducts. Which of the following is the most likely diagnosis? 

A) Acute cholecystitis 

B) Acute pancreatitis

 C) Choledocholithiasis 

D) Hepatitis A 

E) Hepatitis B

 Answer: C. Choledocholithiasis is characterized by abdominal pain, fever, and jaundice, and is confirmed by imaging studies showing dilated intrahepatic and extrahepatic bile ducts.

A 60-year-old male presents with chest pain, shortness of breath, and diaphoresis. Electrocardiogram shows ST-segment elevation in leads II, III, and aVF. Which of the following is the most likely diagnosis? A) Acute myocardial infarction involving the inferior wall 

B) Acute myocardial infarction involving the anterior wall 

C) Acute myocardial infarction involving the lateral wall 

D) Acute pericarditis 

E) Aortic dissection 

Answer: A. ST-segment elevation in leads II, III, and aVF indicates an acute myocardial infarction involving the inferior wall.

A 30-year-old female presents with a 3-day history of progressive weakness, fatigue, and shortness of breath. Laboratory studies reveal microcytic hypochromic anemia, and peripheral blood smear shows target cells and basophilic stippling. Which of the following is the most likely diagnosis? 

A) Iron deficiency anemia 

B) Thalassemia 

C) Sickle cell anemia 

D) Megaloblastic anemia 

E) Hemolytic anemia 

Answer: A. Microcytic hypochromic anemia, target cells, and basophilic stippling are characteristic findings of iron deficiency anemia.

A 65-year-old male with a history of heavy tobacco use presents with hemoptysis, weight loss, and chest pain. Chest X-ray shows a large mass in the right lung. Biopsy reveals small cell carcinoma. Which of the following is the most likely cause of the patient's symptoms? 

A) Tumor invasion of the lung parenchyma

 B) Obstruction of the airways by the tumor 

C) Pulmonary embolism 

D) Pneumonia 

E) Pleural effusion

 Answer: B. Obstruction of the airways by the tumor is the most likely cause of hemoptysis, weight loss, and chest pain in a patient with lung cancer.