A 6-month-old girl born overseas, who had no newborn screening, is brought in for developmental delay. She has eczema, unusually light skin and hair, and her urine has a musty smell. A deficiency of which enzyme is the most likely cause?
Phenylalanine hydroxylase This is phenylketonuria (PKU). Without phenylalanine hydroxylase, phenylalanine builds up and damages the developing brain, and the musty odor comes from its metabolites. Less tyrosine is made, so less melanin is made too, which explains the light skin and hair.
A healthy couple’s first child has cystic fibrosis. Genetic testing shows that both parents are carriers of a disease-causing CFTR variant. What is the chance that their next child will have cystic fibrosis?
25% Cystic fibrosis is autosomal recessive. Each carrier parent passes on the faulty copy half the time, so the chance a child gets two faulty copies is ½ × ½ = 25%. That chance is the same for every pregnancy, no matter what happened before.
A 22-year-old man has had painful urination and a thick yellow-green discharge from his penis for 3 days. A Gram stain of the discharge shows gram-negative diplococci inside neutrophils. Which organism is the most likely cause?
Neisseria gonorrhoeae Gram-negative diplococci inside neutrophils are the classic finding for Neisseria gonorrhoeae. It typically causes urethritis with a thick, purulent discharge. Because chlamydia often comes along with it, patients are usually tested for both.
Within minutes of a bee sting, a 30-year-old woman develops hives, wheezing and a blood pressure of 78/40 mm Hg. Which mechanism best explains her reaction?
Allergen cross-linking IgE on mast cells, which release histamine This is anaphylaxis, a type I hypersensitivity reaction. On a previous exposure she made IgE against the venom, and it sits on mast cells. When the venom cross-links that IgE, the mast cells release histamine and other mediators within minutes, causing hives, bronchospasm and low blood pressure.
A 67-year-old man with atrial fibrillation is started on warfarin. Which blood test is used to monitor his dose?
Prothrombin time (PT) with INR Warfarin blocks vitamin K epoxide reductase, so the liver makes less active factors II, VII, IX and X. Factor VII has the shortest half-life, and it’s part of the extrinsic pathway measured by the PT. The INR standardizes the PT between labs, and doses are adjusted to keep it in a target range.
A 55-year-old woman starts lisinopril for high blood pressure. Three weeks later she comes back with a dry, tickly cough that won’t go away. Her lungs are clear. What is the most likely cause of the cough?
Buildup of bradykinin ACE breaks down bradykinin as well as making angiotensin II. When an ACE inhibitor blocks it, bradykinin builds up in the airways and can cause a dry cough. Switching to an angiotensin receptor blocker usually solves the problem, because ARBs don’t affect bradykinin.
A 40-year-old farmworker is brought in after spraying an organophosphate insecticide. He has pinpoint pupils, heavy sweating and drooling, wheezing, diarrhea and a heart rate of 44/min. Which drug should be given to reverse these muscarinic effects?
Atropine Organophosphates block acetylcholinesterase, so acetylcholine builds up at every cholinergic synapse. Atropine blocks muscarinic receptors and reverses the secretions, bronchospasm and slow heart rate. Pralidoxime is also given to reactivate the enzyme, ideally early, but atropine treats the muscarinic signs.
A 78-year-old man has fainted twice while climbing stairs and gets chest tightness on exertion. He has a harsh crescendo-decrescendo systolic murmur at the right upper sternal border that radiates to both carotid arteries. Which valve problem is most likely?
Aortic stenosis A crescendo-decrescendo systolic murmur at the right upper sternal border that radiates to the carotids is the textbook sign of aortic stenosis. In older adults it’s usually from calcification. Syncope, angina and shortness of breath on exertion are the classic symptoms, and once they appear the valve usually needs replacing.
A 25-year-old man is brought in after a car crash. His blood pressure is 78/40 mm Hg and his heart rate is 132/min. His neck veins are distended, his trachea is shifted to the right, and the left side of his chest has no breath sounds and is hyperresonant. What is the most appropriate next step?
Immediate needle decompression of the left chest This is a tension pneumothorax, and it’s diagnosed at the bedside. Air trapped in the left chest pushes the mediastinum over and stops blood returning to the heart, which causes shock. Decompress it right away, then place a chest tube. Waiting for imaging could cost him his life.
A 28-year-old woman at 35 weeks of her first pregnancy has a headache. Her blood pressure is 164/112 mm Hg on two readings 15 minutes apart, and her urine shows 2+ protein. Along with an antihypertensive, which drug should be given to prevent seizures?
Magnesium sulfate She has preeclampsia with severe features: blood pressure of 160/110 or higher plus a headache. Magnesium sulfate is the drug of choice to prevent eclamptic seizures, and it also treats them if they occur. Delivery is the definitive treatment.
A 19-year-old woman with type 1 diabetes has had vomiting and abdominal pain for a day. Her glucose is 520 mg/dL, arterial pH is 7.15, bicarbonate is 10 mEq/L, potassium is 4.8 mEq/L, and her serum ketones are high. What is the most appropriate first step in management?
IV 0.9% saline This is diabetic ketoacidosis. Patients are badly dehydrated, so the first step is IV isotonic fluid. An IV insulin infusion follows, once potassium is known to be at least 3.3 mEq/L, with potassium replaced as needed. Bicarbonate isn’t routine at this pH.
A 32-year-old woman has lost weight despite a good appetite. She feels hot all the time and has palpitations. Her eyes bulge, her thyroid is diffusely enlarged and not tender, and the skin over her shins is thickened. TSH is low and free T4 is high. What is the cause of her condition?
Antibodies that stimulate the TSH receptor This is Graves disease. Antibodies bind the TSH receptor and switch it on, so the whole gland grows and makes too much hormone. Bulging eyes and thickened skin on the shins (pretibial myxedema) are specific to Graves and don’t happen with other causes of hyperthyroidism.
A 4-year-old boy has had puffy eyes and swollen legs for a week. His blood pressure is normal. Urinalysis shows 4+ protein and no blood. His serum albumin is low and his cholesterol is high. What is the most likely diagnosis?
Minimal change disease Heavy protein in the urine, low albumin, edema and high cholesterol make up nephrotic syndrome. In young children the most common cause is minimal change disease. The kidney looks normal on light microscopy, electron microscopy shows effaced podocyte foot processes, and it usually responds well to corticosteroids.
A 29-year-old woman with very heavy periods has felt tired for months. Her hemoglobin is 9.8 g/dL and her mean corpuscular volume (MCV) is 70 fL. Which other results are most likely?
Low ferritin and high total iron-binding capacity Chronic blood loss from heavy periods causes iron deficiency, the most common cause of a microcytic anemia. Ferritin, the body’s iron store, is low. The liver makes more transferrin to grab whatever iron it can, so the total iron-binding capacity (TIBC) goes up.
A 45-year-old woman has had constant right upper abdominal pain for 8 hours, starting after a fatty meal. Her temperature is 38.4°C (101.1°F), and she stops breathing in when the right upper quadrant is pressed. Her white cell count is 14,000/mm³ and her bilirubin is normal. What is the best initial imaging test?
Right upper quadrant ultrasound This looks like acute cholecystitis: right upper quadrant pain, fever, a raised white count and a positive Murphy sign. Ultrasound is the best first test. It’s quick, uses no radiation, and can show gallstones, a thick gallbladder wall and fluid around the gallbladder.
A 70-year-old right-handed man suddenly develops weakness of the right side of his face and his right arm. His right leg is only mildly weak. He understands what’s said to him but struggles to get words out. Which artery is most likely blocked?
Left middle cerebral artery Weakness of the face and arm more than the leg points to the middle cerebral artery territory. Symptoms on the right side mean the problem is in the left hemisphere. Trouble producing speech is expressive (Broca) aphasia, and language sits in the left hemisphere in most right-handed people.
A 66-year-old woman arrives at the emergency department 1 hour after she suddenly lost strength in her left arm and leg. Her blood pressure is 168/94 mm Hg and her fingerstick glucose is 112 mg/dL. What is the most appropriate next step?
Noncontrast CT scan of the head Before any treatment for a possible stroke, you have to rule out bleeding. A noncontrast head CT does that quickly. If there’s no bleed and she qualifies, clot-busting treatment can follow within the time window.
A 34-year-old man has had low mood for 6 weeks, with poor sleep, low energy, trouble concentrating and no interest in things he used to enjoy. He has no thoughts of suicide, has never had a manic episode and doesn’t use alcohol or drugs. He would like to try medication. Which is the most appropriate first choice?
Sertraline He meets the criteria for major depressive disorder. A selective serotonin reuptake inhibitor (SSRI) such as sertraline is a first-line medication because it works and is generally well tolerated and safer in overdose than older options. It can take several weeks to work fully.
A new blood test is studied in 1,000 people. Of the 200 who have the disease, 180 test positive. Of the 800 who don’t have it, 760 test negative. What is the sensitivity of the test?
90% Sensitivity is the share of people with the disease who test positive: true positives divided by everyone who has the disease. Here that’s 180 ÷ 200 = 90%.
A 40-year-old woman who is a Jehovah’s Witness is admitted with a bleeding stomach ulcer. Her hemoglobin is 6.8 g/dL. She’s alert and oriented, understands that she could die without blood, and clearly refuses a transfusion. What should the physician do?
Respect her refusal and continue other treatment An adult with decision-making capacity can refuse any treatment, even one that could save her life. The physician should respect the refusal, document the discussion, and treat the bleeding in every other way she accepts, such as endoscopy, IV fluids and medicines.
United States · Licensing exam