USMLE Step 1 Practice Questions and Answers

A 38-year-old man presents to the emergency department due to seizures that started earlier that day, as reported by his sister. He adds that his vision is also blurry. The patient says he has never traveled outside of the United States. Physical examination reveals several enlarged cervical lymph nodes as well as a right homonymous hemianopia. Laboratory studies show a CD4 count of 78 cells/μL. The patient is sent for an MRI with ring-enhancing lesion. - Toxoplasma Gondii Occurs in patients with HIV whose CD4 count is 100 cells/μL. It most commonly manifests with encephalitis characterized by seizures and/or focal neurologic deficits. The classic picture on a CT scan or an MRI of the head is a ring-enhancing lesion with surrounding edema and mass effect (as shown in this MRI). The specific visual field defect in this patient is due to the cerebral lesions causing a focal neurologic deficit. New-onset seizures in an HIV-positive patient with a CD4 count 100 cells/µL is highly suggestive of toxoplasmosis. Other manifestations are chorioretinitis, which is characterized by eye pain and decreased visual acuity, as seen in this patient. A 34-year-old man presents to the emergency department with a dry cough, low-grade fevers, and difficulty breathing that becomes progressively worse with exertion. He says that he has had these symptoms for the past 2-3 weeks. He reports feeling increasingly fatigued and has had an unintentional weight loss of 10 pounds (4.5 kilograms) in the same time period. The patient denies any alcohol, tobacco, or illicit drug use but admits to having had multiple sexual partners without using protection. The patient's temperature is 38.1oC (100.6oF), blood pressure is 102/72 mm Hg, pulse is 96/min, respiratory rate is 22/min, and SpO2 is 86% on room air. Physical examination shows oral thrush, bilateral rales at the lung bases, and right axillary and left inguinal lymphadenopathy. - Inhibition of Folate Synthesis This patient presents with fever, dyspnea on exertion, and unintentional weight loss, which are suggestive of pneumonia. He also has a history of unprotected sex with multiple partners, and presents with oral thrush and inguinal lymphadenopathy, which are suggestive of HIV. Taken together, the patient's presentation and history raise suspicion of an opportunistic infection, such as Pneumocystis jirovecii (PCP) pneumonia. PCP pneumonia is an AIDS-defining illness and is typically seen in patients with CD4 counts 200 cells/mm. P. jirovecii is an ascomycetous fungi that also causes pneumonia in hematopoietic-cell and solid-organ transplant recipients, cancer patients, and patients receiving glucocorticoids, chemotherapeutic agents, and other immunosuppressive medications. The standard treatment for this illness is trimethoprim-sulfamethoxazole (TMP-SMX), a combination antibiotic that synergistically inhibits folate synthesis by inhibiting dihydrofolate reductase and dihydropteroate synthetase. If the patient has a sulfa-drug allergy, the treatment of choice would be pentamidine. A previously healthy 5-year-old boy is brought to the pediatrician with a 3-day history of sore throat, conjunctivitis, rhinitis, and cough. His mother explains that more than 10 children in his class at school have similar symptoms, particularly conjunctivitis. His temperature is 100.4°F (38°C), pulse is 108/min, and blood pressure is 110/80 mm Hg. Physical examination shows the finding in the image. No cultures are ordered, and the mother is assured that her son's illness will go away on its own. One week later, the mother reports that her son is healthy and back at school. - This patient presents with many symptoms of a viral upper respiratory infection (rhinitis, sore throat, cough) and the finding of conjunctival infection or "pink eye." He is also one of many children in his class to have such symptoms. He likely has an adenovirus infection, which is a major cause of epidemic keratoconjunctivitis (pink eye). It is the fourth most common cause of childhood respiratory tract infections after respiratory syncytial virus, parainfluenza, and rhinovirus. It is a naked, icosahedral, double-stranded linear DNA virus that results in a self-limited illness that requires no treatment. Although rhinovirus (a cause of the "common cold") would cause many of the symptoms the child is experiencing, it is less likely to cause the pink eye seen in this patient.