Physical exam-directed testing

Core Principles - Examine, Then Order!

Principle: Physical exam findings guide test selection by modifying pre-test probability.
Goal: Avoid the inefficient "shotgun approach" of ordering numerous non-specific tests.
- This ↑ diagnostic yield, ↓ costs, and minimizes patient harm.

⭐ A test is most useful when the pre-test probability is intermediate (~30-70%). Strong clinical findings can move the probability to be very high or very low, often reducing the need for further testing.

Cardiovascular System - Heartfelt Investigations

Murmur: Echocardiogram (first-line).
Pericardial Rub: ECG (widespread ST elevation), then Echocardiogram.
S3 Gallop / Dyspnea: Pro-BNP, Echocardiogram, Chest X-ray (cardiomegaly).
Unequal Pulses / BP / Tearing Chest Pain: CT Angiogram (rule out aortic dissection).
Radio-femoral Delay: Doppler, CT/MR Angiogram (coarctation).

Echocardiogram of mitral valve prolapse with measurements

⭐ Beck's Triad for Cardiac Tamponade: Hypotension, Distended Neck Veins (JVD), and Muffled Heart Sounds. A clinical diagnosis prompting urgent pericardiocentesis.

Respiratory System - Breathing Easy with Tests

Link physical findings directly to your initial test choice. This targeted approach saves time and resources, focusing on the most probable diagnosis.

Dullness to percussion: Indicates fluid or consolidation.
- Initial Test: Chest X-ray (CXR) to identify pleural effusion or lobar pneumonia.
Hyper-resonant note: Suggests trapped air.
- Initial Test: CXR to look for pneumothorax.
Crackles (Rales): Implies fluid in alveoli.
- Initial Test: CXR. Consider Brain Natriuretic Peptide (BNP) if suspecting a cardiac origin.
Wheeze: Airway narrowing.
- Initial Test: Spirometry / Pulmonary Function Test (PFT).
Stridor: Upper airway obstruction.
- Initial Test: Laryngoscopy or X-ray of the neck (lateral view).

⭐ On auscultation, egophony ("E" to "A" change) has a high specificity for consolidation.

Abdominal & GI Exam - Gut Feelings to Certainty

Translating physical signs into diagnostic action is key. Specific abdominal maneuvers point towards high-probability diagnoses, guiding the most appropriate initial imaging or lab tests.

Sign	Suspected Diagnosis	Best Initial Test
Murphy's Sign	Acute Cholecystitis	USG Abdomen
McBurney's, Rovsing's, Psoas, Obturator	Acute Appendicitis	USG Abdomen / CT
Cullen's, Grey Turner's	Hemorrhagic Pancreatitis & Retroperitoneal Bleed	CT Abdomen, Amylase/Lipase
Caput Medusae	Portal Hypertension	LFTs, USG Doppler

Neurological Exam - Nerve-Wracking Decisions

Your physical exam is pivotal. Neurological red flags mandate immediate imaging to exclude life-threatening conditions like mass effect or ↑ICP before proceeding to procedures like a lumbar puncture. This sequence prevents iatrogenic herniation.

⭐ In suspected meningitis with signs of ↑ICP (papilledema, focal deficits), always get a CT head before LP to prevent brainstem herniation.

High-Yield Points - ⚡ Biggest Takeaways

Physical examination is a diagnostic filter, not just a ritual; use it to guide your investigations.

Order tests to confirm or refute diagnoses suggested by specific physical signs.

A new cardiac murmur directly indicates the need for an ECG and 2D Echocardiogram.

Abdominal signs like guarding or tenderness should prompt targeted imaging like ultrasound or CT.

Focal neurological deficits are a clear indication for immediate neuroimaging (CT/MRI).

This sign-to-test strategy is crucial for efficient diagnosis in CCS cases.

Physical exam-directed testing US Medical PG Practice Questions and MCQs

Practice US Medical PG questions for Physical exam-directed testing. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.

Physical exam-directed testing US Medical PG Question 1: A scientist in Boston is studying a new blood test to detect Ab to the parainfluenza virus with increased sensitivity and specificity. So far, her best attempt at creating such an exam reached 82% sensitivity and 88% specificity. She is hoping to increase these numbers by at least 2 percent for each value. After several years of work, she believes that she has actually managed to reach a sensitivity and specificity even greater than what she had originally hoped for. She travels to South America to begin testing her newest blood test. She finds 2,000 patients who are willing to participate in her study. Of the 2,000 patients, 1,200 of them are known to be infected with the parainfluenza virus. The scientist tests these 1,200 patients’ blood and finds that only 120 of them tested negative with her new test. Of the following options, which describes the sensitivity of the test?

A. 82%
B. 86%
C. 98%
D. 90% (Correct Answer)
E. 84%

Physical exam-directed testing Explanation: ***90%*** - **Sensitivity** is calculated as the number of **true positives** divided by the total number of individuals with the disease (true positives + false negatives). - In this scenario, there were 1200 infected patients (total diseased), and 120 of them tested negative (false negatives). Therefore, 1200 - 120 = 1080 patients tested positive (true positives). The sensitivity is 1080 / 1200 = 0.90, or **90%**. *82%* - This value was the **original sensitivity** of the test before the scientist improved it. - The question states that the scientist believes she has achieved a sensitivity "even greater than what she had originally hoped for." *86%* - This value is not directly derivable from the given data for the new test's sensitivity. - It might represent an intermediate calculation or an incorrect interpretation of the provided numbers. *98%* - This would imply only 24 false negatives out of 1200 true disease cases, which is not the case (120 false negatives). - A sensitivity of 98% would be significantly higher than the calculated 90% and the initial stated values. *84%* - This value is not derived from the presented data regarding the new test's performance. - It could be mistaken for an attempt to add 2% to the original 82% sensitivity, but the actual data from the new test should be used.

Physical exam-directed testing US Medical PG Question 2: A 70-year-old male visits his primary care physician because of progressive weight loss. He has a 20-year history of smoking 2 packs of cigarettes a day and was diagnosed with diabetes mellitus 6 years ago. After physical examination, the physician tells the patient he suspects adenocarcinoma at the head of the pancreas. Which of the following physical examination findings would support the diagnosis?

A. Lymphadenopathy of the umbilicus
B. Splenomegaly
C. Acanthosis nigricans
D. Palpable gallbladder (Correct Answer)
E. Renal artery bruits

Physical exam-directed testing Explanation: ***Palpable gallbladder*** - A palpable, non-tender gallbladder in the presence of **jaundice** (often caused by obstruction of the common bile duct) is known as **Courvoisier's sign**, which is highly suggestive of pancreatic head adenocarcinoma. - The tumor in the head of the pancreas can compress the distal common bile duct, leading to bile stasis and gallbladder distension. *Lymphadenopathy of the umbilicus* - **Umbilical lymphadenopathy**, specifically a **Sister Mary Joseph nodule**, indicates distant metastatic disease, often from intra-abdominal cancers like gastrointestinal or ovarian. - While it points to an advanced malignancy, it is not a direct physical finding specific for initial suspicion of primary pancreatic head adenocarcinoma. *Splenomegaly* - **Splenomegaly** can be a feature of pancreatic cancer if there is splenic vein thrombosis due to tumor invasion, leading to **portal hypertension**. - However, it is not a primary sign of pancreatic head adenocarcinoma and typically occurs in more advanced or specific cases. *Acanthosis nigricans* - **Acanthosis nigricans** is characterized by hyperpigmented, velvety plaques, often in skin folds, and can be a paraneoplastic syndrome associated with various malignancies, including pancreatic cancer. - While possible, it is a less direct and less specific sign for pancreatic head adenocarcinoma compared to Courvoisier's sign. *Renal artery bruits* - **Renal artery bruits** indicate turbulent blood flow through the renal arteries, most commonly due to **renal artery stenosis**, which can cause hypertension. - This finding is unrelated to pancreatic adenocarcinoma and would not support such a diagnosis.

Physical exam-directed testing US Medical PG Question 3: A research group wants to assess the safety and toxicity profile of a new drug. A clinical trial is conducted with 20 volunteers to estimate the maximum tolerated dose and monitor the apparent toxicity of the drug. The study design is best described as which of the following phases of a clinical trial?

A. Phase 0
B. Phase III
C. Phase V
D. Phase II
E. Phase I (Correct Answer)

Physical exam-directed testing Explanation: ***Phase I*** - **Phase I clinical trials** involve a small group of healthy volunteers (typically 20-100) to primarily assess **drug safety**, determine a safe dosage range, and identify side effects. - The main goal is to establish the **maximum tolerated dose (MTD)** and evaluate the drug's pharmacokinetic and pharmacodynamic profiles. *Phase 0* - **Phase 0 trials** are exploratory studies conducted in a very small number of subjects (10-15) to gather preliminary data on a drug's **pharmacodynamics and pharmacokinetics** in humans. - They involve microdoses, not intended to have therapeutic effects, and thus cannot determine toxicity or MTD. *Phase III* - **Phase III trials** are large-scale studies involving hundreds to thousands of patients to confirm the drug's **efficacy**, monitor side effects, compare it to standard treatments, and collect information that will allow the drug to be used safely. - These trials are conducted after safety and initial efficacy have been established in earlier phases. *Phase V* - "Phase V" is not a standard, recognized phase in the traditional clinical trial classification (Phase 0, I, II, III, IV). - This term might be used in some non-standard research contexts or for post-marketing studies that go beyond Phase IV surveillance, but it is not a formal phase for initial drug development. *Phase II* - **Phase II trials** involve several hundred patients with the condition the drug is intended to treat, focusing on **drug efficacy** and further evaluating safety. - While safety is still monitored, the primary objective shifts to determining if the drug works for its intended purpose and at what dose.

Physical exam-directed testing US Medical PG Question 4: A 57-year-old man was brought into the emergency department unconscious 2 days ago. His friends who were with him at that time say he collapsed on the street. Upon arrival to the ED, he had a generalized tonic seizure. At that time, he was intubated and is being treated with diazepam and phenytoin. A noncontrast head CT revealed hemorrhages within the pons and cerebellum with a mass effect and tonsillar herniation. Today, his blood pressure is 110/65 mm Hg, heart rate is 65/min, respiratory rate is 12/min (intubated, ventilator settings: tidal volume (TV) 600 ml, positive end-expiratory pressure (PEEP) 5 cm H2O, and FiO2 40%), and temperature is 37.0°C (98.6°F). On physical examination, the patient is in a comatose state. Pupils are 4 mm bilaterally and unresponsive to light. Cornea reflexes are absent. Gag reflex and cough reflex are also absent. Which of the following is the next best step in the management of this patient?

A. Second opinion from a neurologist
B. Withdraw ventilation support and mark time of death
C. Electroencephalogram
D. Repeat examination in several hours
E. Apnea test (Correct Answer)

Physical exam-directed testing Explanation: ***Apnea test*** - The patient exhibits classic signs of **brain death**, including a **coma**, fixed and dilated pupils, and absent brainstem reflexes (corneal, gag, cough). The next step is to perform an apnea test to confirm the absence of spontaneous respiratory drive. - An apnea test confirms brain death by demonstrating the **absence of respiratory effort** despite a rising pCO2, provided that spinal cord reflexes are not mistaken for respiratory efforts. *Second opinion from a neurologist* - While consulting a neurologist is often helpful in complex neurological cases, the current clinical picture presents such clear signs of brain death that **further confirmatory testing** for brain death (like the apnea test) is more immediately indicated before seeking additional opinions on diagnosis. - A second opinion would typically be sought to confirm the diagnosis or guide management, but establishing brain death requires a specific protocol which is incomplete without the apnea test. *Withdraw ventilation support and mark time of death* - It is **premature to withdraw ventilation** before brain death is unequivocally confirmed by all necessary clinical and confirmatory tests, including the apnea test. - Withdrawing support without full confirmation could lead to ethical and legal issues, as the patient might still have residual brainstem function, however minimal. *Electroencephalogram* - An **EEG** can show absent electrical activity, supporting brain death, but it is **not a mandatory part of the core brain death criteria** in many protocols, especially when clinical signs are clear and an apnea test can be performed. - The primary diagnostic criteria for brain death usually prioritize clinical examination and the apnea test for proving irreversible cessation of all brain functions. *Repeat examination in several hours* - Repeating the examination in several hours is typically done if there are **confounding factors** (e.g., severe hypothermia, drug intoxication) that might mimic brain death, or if the initial assessment is incomplete. - In this case, there are no mentioned confounding factors, and the immediate priority is to complete the brain death protocol with an apnea test, given the current clear clinical picture.

Physical exam-directed testing US Medical PG Question 5: A 65-year-old man presents to his primary-care doctor for a 2-month history of dizziness. He describes feeling unsteady on his feet or like he's swaying from side-to-side; he's also occasionally had a room-spinning sensation. He first noticed it when he was in the front yard playing catch with his grandson, and he now also reliably gets it when throwing the frisbee with his dog. The dizziness only happens during these times, and it goes away after a couple of minutes of rest. His medical history is notable for type 2 diabetes mellitus treated with metformin. His vital signs are within normal limits in the office. The physical exam is unremarkable. Which of the following is the next best test for this patient?

A. Ankle-brachial index
B. Doppler ultrasound (Correct Answer)
C. Transthoracic echocardiogram
D. CT head (noncontrast)
E. Electrocardiogram

Physical exam-directed testing Explanation: ***Doppler ultrasound*** - The patient's dizziness occurring specifically during **upper extremity exertion** (playing catch, throwing frisbee) that resolves with rest is classic for **subclavian steal syndrome**. - In subclavian steal, stenosis of the subclavian artery proximal to the vertebral artery origin causes **reversed flow in the vertebral artery** during arm exercise, "stealing" blood from the posterior circulation and causing vertebrobasilar insufficiency symptoms. - **Doppler ultrasound** of the subclavian and vertebral arteries is the appropriate initial test to demonstrate reversed vertebral flow and subclavian stenosis. This can be confirmed with provocative maneuvers (arm exercise during the study). - The patient's diabetes increases his risk for atherosclerotic disease, making this diagnosis more likely. *CT head (noncontrast)* - While CT head can evaluate for stroke or structural brain lesions, it would **not visualize the vascular stenosis** causing subclavian steal syndrome. - The episodic nature triggered by specific arm movements and complete resolution with rest makes an acute structural brain lesion unlikely. - CT head would be appropriate if symptoms were persistent, progressive, or associated with focal neurological deficits. *Ankle-brachial index* - ABI is used to diagnose **peripheral artery disease** affecting the lower extremities, typically presenting with **intermittent claudication** (leg pain with walking). - This does not evaluate the upper extremity or cerebrovascular circulation relevant to this patient's symptoms. *Electrocardiogram* - ECG evaluates cardiac rhythm and ischemia but would not diagnose the **vascular steal phenomenon** causing his symptoms. - Cardiac arrhythmias typically cause syncope or presyncope rather than positional dizziness triggered by specific arm movements. - While arrhythmias should be considered in the differential, the clear association with arm exercise points to a vascular steal phenomenon. *Transthoracic echocardiogram* - TTE assesses cardiac structure and function (valves, chambers, ejection fraction) but does not evaluate the **extracranial vasculature**. - Cardiac causes of dizziness (e.g., severe aortic stenosis) would more likely present with exertional syncope rather than dizziness specifically with arm movements. - The symptom pattern does not suggest primary cardiac pathology.

Physical exam-directed testing US Medical PG Question 6: A 33-year-old man presents to his primary care physician with shoulder pain. He states that he can't remember a specific instance when the injury occurred. He is a weight lifter and competes in martial arts. The patient has no past medical history and is currently taking a multivitamin. Physical exam demonstrates pain with abduction of the patient's right shoulder and with external rotation of the right arm. There is subacromial tenderness with palpation. His left arm demonstrates 10/10 strength with abduction as compared to 4/10 strength with abduction of the right arm. Which of the following best confirms the underlying diagnosis?

A. Ultrasound
B. Radiography
C. MRI (Correct Answer)
D. CT
E. Physical exam and history

Physical exam-directed testing Explanation: ***MRI*** - An **MRI is the gold standard** for diagnosing soft tissue injuries of the shoulder, including **rotator cuff pathology**, which is highly suspected given the patient's symptoms (pain with abduction and external rotation, subacromial tenderness, and weakness). - It provides detailed imaging of tendons, ligaments, and cartilage, allowing for precise identification of **tears, inflammation, or impingement**. *Ultrasound* - While ultrasound can assess **rotator cuff integrity** and identify fluid collections, it is highly operator-dependent and may not provide the same level of detail as MRI for complex tears or associated pathologies. - It can be a good initial screening tool but might **underestimate the extent** of an injury compared to MRI. *Radiography* - **Radiography (X-rays)** primarily visualizes bone structures and would be useful for detecting fractures, dislocations, or significant degenerative joint disease. - It would **not directly visualize** the soft tissue injuries of the rotator cuff or other tendons that are likely causing this patient's symptoms. *CT* - **CT scans** provide excellent detail of bone structures and can identify subtle fractures, erosions, or bony impingement. - However, like X-rays, they are **less effective for visualizing soft tissues** like tendons and ligaments compared to MRI. *Physical exam and history* - The **physical exam and history** are crucial for narrowing down the differential diagnosis and guiding further imaging. - While strongly suggestive of a rotator cuff injury, they alone **cannot definitively confirm the extent or nature** of the underlying soft tissue pathology.

Physical exam-directed testing US Medical PG Question 7: A 16-year-old presents to the primary care physician because he has noticed an increase in the size of his breast tissue over the past 3 years. He states that he is significantly taller than his entire class at school although he feels increasingly weak and uncoordinated. He performs at the bottom of his grade level academically. On physical exam the patient has marked gynecomastia with small firm testes. The physician decides to perform a karyotype on the patient. What is the most likely outcome of this test?

A. 47, XYY
B. 45, XO
C. 47, XXY (Correct Answer)
D. 47, XXX
E. 46, XY

Physical exam-directed testing Explanation: ***47, XXY*** The constellation of **gynecomastia, tall stature, learning difficulties, and small testes** is classic for **Klinefelter syndrome**, which is characterized by a **47, XXY** karyotype. The presence of an extra X chromosome leads to **hypogonadism** and **endocrine imbalances**, explaining the physical and developmental findings. *47, XYY* - Individuals with **47, XYY syndrome** are typically tall but do not usually present with breast enlargement or other features of hypogonadism. - They may have an increased risk of learning difficulties and behavioral problems. *45, XO* - This karyotype, also known as **Turner syndrome**, is characterized by the absence of an X chromosome and typically affects **females**. - Common features include **short stature, webbed neck, and ovarian dysfunction**, which are inconsistent with the patient's presentation. *47, XXX* - This karyotype, known as **Triple X syndrome**, affects **females** and is characterized by the presence of an extra X chromosome. - While some individuals may experience learning difficulties or developmental delays, it does not cause gynecomastia or small testes in males. *46, XY* - This is the **normal male karyotype** and would not explain the patient's symptoms of gynecomastia, small testes, tall stature, or learning difficulties. - These symptoms suggest an underlying chromosomal abnormality.

Physical exam-directed testing US Medical PG Question 8: A 60-year-old-man presents to his physician with worsening myalgias and new symptoms of early fatigue, muscle weakness, and drooping eyelids. His wife presents with him and states that he never used to have such symptoms. His medical history is significant for gout, hypertension, hypercholesterolemia, diabetes mellitus type II, and pilocytic astrocytoma as a teenager. He denies smoking, drinks a 6-pack of beer per day, and endorses a past history of cocaine use but currently denies any illicit drug use. His vital signs include temperature 36.7°C (98.0°F), blood pressure 126/74 mm Hg, heart rate 87/min, and respiratory rate 15/min. Physical examination shows minimal bibasilar rales, but otherwise clear lungs on auscultation, grade 2/6 holosystolic murmur, 3/5 strength in all extremities, and benign abdominal findings. The Tensilon test result is positive. Which of the following options explains why a chest CT should be ordered for this patient?

A. Evaluation for mediastinal botulinum abscess
B. Evaluation of congenital vascular anomaly
C. Exclusion of underlying lung cancer
D. Assessment for motor neuron disease
E. Exclusion of a thymoma (Correct Answer)

Physical exam-directed testing Explanation: ***Exclusion of a thymoma*** - The positive **Tensilon test** strongly indicates **myasthenia gravis**, a condition frequently associated with **thymoma**, a tumor of the thymus gland. - A **chest CT** is crucial for identifying or excluding a **thymoma** in patients with myasthenia gravis, as its resection can improve symptoms. *Evaluation for mediastinal botulinum abscess* - **Botulism** would present with descending paralysis and autonomic dysfunction, and an abscess is not a typical manifestation or diagnostic consideration in this context. - While mediastinal abscesses can occur, they are usually associated with infection, trauma, or surgery and not directly linked to the patient's symptoms or positive Tensilon test. *Evaluation of congenital vascular anomaly* - Congenital vascular anomalies are typically diagnosed earlier in life and are not directly associated with the new onset of myalgias, fatigue, muscle weakness, and ptosis in a 60-year-old. - There are no clinical signs or symptoms presented that would suggest a vascular anomaly as a cause for his current presentation. *Exclusion of underlying lung cancer* - Although lung cancer can cause **paraneoplastic syndromes** such as Lambert-Eaton myasthenic syndrome, the symptoms of myalgias, muscle weakness, and ptosis improving with Tensilon strongly point to **myasthenia gravis**, not a paraneoplastic process. - The specific combination of symptoms and a positive Tensilon test makes myasthenia gravis and its association with thymoma a more direct concern requiring evaluation. *Assessment for motor neuron disease* - **Motor neuron diseases** (e.g., ALS) typically present with progressive muscle weakness, spasticity, and fasciculations, but they do not show improvement with a **Tensilon test**. - The positive Tensilon test specifically rules against motor neuron disease as the primary cause for the reported symptoms.

Physical exam-directed testing US Medical PG Question 9: A 28-year-old woman comes to the emergency department for a rash that began 3 days ago. She has low-grade fever and muscle aches. She has no history of serious illness and takes no medications. She has had 5 male sexual partners over the past year and uses condoms inconsistently. Her temperature is 38.1°C (100.6° F), pulse is 85/min, and blood pressure is 126/89 mm Hg. Examination shows a diffuse maculopapular rash that includes the palms and soles. The remainder of the examination shows no abnormalities. A venereal disease research laboratory (VDRL) test is positive. Which of the following is the next appropriate step in management?

A. Intravenous penicillin G
B. Dark field microscopy
C. Treponemal culture
D. Oral doxycycline
E. Fluorescent treponemal antibody absorption test (Correct Answer)

Physical exam-directed testing Explanation: ***Fluorescent treponemal antibody absorption test*** - A positive **VDRL** (a non-treponemal test) should be confirmed with a **treponemal-specific test** like the **fluorescent treponemal antibody absorption (FTA-ABS)** test or **Treponema pallidum particle agglutination (TPPA)** assay to definitively diagnose syphilis. - This confirmatory step helps differentiate true syphilis from false-positive VDRL results, which can occur in autoimmune diseases (SLE, antiphospholipid syndrome), other infections (malaria, mononucleosis), pregnancy, or recent vaccination. - While this patient's presentation is highly suggestive of **secondary syphilis** (diffuse maculopapular rash involving palms and soles, fever, myalgias, positive VDRL), confirmatory testing is the standard next step before initiating treatment. - Note: In some clinical settings with classic secondary syphilis, immediate treatment may be initiated, but confirmatory testing remains the most appropriate next diagnostic step. *Intravenous penicillin G* - IV aqueous penicillin G is the treatment for **neurosyphilis**, not uncomplicated secondary syphilis. - **Secondary syphilis** is treated with **intramuscular benzathine penicillin G 2.4 million units** as a single dose. - Treatment should follow confirmed diagnosis with treponemal-specific testing. *Dark field microscopy* - This technique visualizes spirochetes directly from **primary lesions** (chancres) or moist secondary lesions (condyloma lata, mucous patches). - It is not practical for this patient who has a diffuse maculopapular rash without obvious mucosal or genital lesions. - Dark-field microscopy requires specialized equipment and expertise not readily available in most emergency departments. *Treponemal culture* - **Treponema pallidum** cannot be cultured on artificial media because it is an **obligate pathogen** that requires living host cells. - Culture is not a diagnostic option for syphilis. *Oral doxycycline* - **Doxycycline 100 mg twice daily for 14 days** is an alternative treatment for early syphilis in **penicillin-allergic patients**. - Treatment should only be initiated after diagnosis is confirmed with treponemal-specific testing. - This is not the next appropriate step; confirmatory testing comes first.

Physical exam-directed testing US Medical PG Question 10: A 28-year-old man presents with one week of redness and discharge in his eyes, pain and swelling in his left second and third toes, and rash on the soles of his feet. He is sexually active with multiple partners and uses condoms occasionally. He denies any recent travel or illness and does not take any medications. Review of systems is otherwise unremarkable. On physical exam, he has bilateral conjunctivitis, dactylitis of the left second and third toes, and crusty yellow-brown vesicles on his plantar feet. Complete blood count and chemistries are within normal limits. Erythrocyte sedimentation rate (ESR) is 40 mm/h. Toe radiographs demonstrate soft tissue swelling but no fractures. Which diagnostic test should be performed next?

A. Rheumatoid factor
B. Anti-cyclic citrullinated peptide antibody assay
C. HLA-B27
D. Nucleic acid amplification testing for Chlamydia trachomatis (Correct Answer)
E. Antinuclear antibody assay

Physical exam-directed testing Explanation: ***Nucleic acid amplification testing for Chlamydia trachomatis*** - The patient's symptoms (conjunctivitis, dactylitis, and plantar rash) with a history of unprotected sexual activity are highly suggestive of **reactive arthritis**, often triggered by **Chlamydia trachomatis infection**. - **NAAT** is the most sensitive and specific test for detecting *Chlamydia trachomatis* in urethral or urine samples, even in asymptomatic infections. *Rheumatoid factor* - This test is primarily used for **rheumatoid arthritis**, which typically presents with symmetric polyarthritis, not the asymmetric dactylitis and conjunctivitis seen here. - The patient’s acute presentation and specific rash are inconsistent with classic rheumatoid arthritis. *Anti-cyclic citrullinated peptide antibody assay* - Like rheumatoid factor, this assay is a specific marker for **rheumatoid arthritis** and would not be the most appropriate first step in diagnosing reactive arthritis. - Its utility in this clinical context is minimal given the patient's symptoms point away from rheumatoid arthritis. *HLA-B27* - While **HLA-B27** is associated with reactive arthritis (and other spondyloarthropathies), it is a **genetic predisposition factor**, not a diagnostic test for acute infection. - A positive result would confirm susceptibility but would not identify the underlying *Chlamydia* infection that needs treatment. *Antinuclear antibody assay* - This assay is used to screen for **systemic lupus erythematosus** and other autoimmune connective tissue diseases. - The patient's acute presentation of conjunctivitis, dactylitis, and plantar rash is not typical for lupus or similar conditions.

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