Special Topics & Strategy System
Special Topics
High-yield cross-cutting content for USMLE Step 1/2/3 — test-taking strategy, high-yield integrated multi-system vignettes, autosomal dominant genetics, basic science integration for Step 2/3, and geriatric population pearls. Organized for maximum exam leverage.
Test-Taking Strategy
The pathway to a high Step 2/3 score is not about memorizing more facts — it is about understanding mechanisms and reading NBME question stems with the right framework. Divine emphasizes that straight answers have largely disappeared from modern USMLE exams; derivative, mechanism-based answers are the norm. Every correction in the Divine Patch represents a commonly missed nuance.
EP573
The Pathway to a High Score on Step 2
- Straight answers are extinct: Modern NBME exams give derivative answers — they name the mechanism, consequence, or related finding rather than the diagnosis. If you memorize labels without understanding mechanisms, you will be stuck. You need to derive the answer from what you understand
- Questions cluster by mechanism, not organ: A question about beta-1 receptors on cardiac myocytes connects hypothyroidism (fewer receptors → bradycardia) to catecholamine pharmacology to obstructive sleep apnea complications. Train yourself to see these chains
- Integration over isolation: When studying, always ask "why does this happen?" and "what else shares this mechanism?" Isolated fact recall fails on Step 2. Pathophysiologic chains succeed
- Past medical history is a clue: When an NBME question lists a prior autoimmune disease, there is a ≥50% chance the current problem is also autoimmune. Autoimmune diseases cluster. Celiac + new amenorrhea → think Hashimoto's, not primary ovarian failure first
- Risk factors are first-line answers: NBME loves risk factor questions. If "age" and a specific modifiable risk factor are both listed as answers for the biggest risk factor, pick the specific one (e.g., atypical ductal hyperplasia beats age for breast cancer if biopsy-proven)
- USPSTF vs ACS on USMLE: When screening guidelines conflict, USMLE generally follows American Cancer Society → mammography starting at age 40. USPSTF (start at 50) is less commonly used as the correct answer for when to begin breast cancer screening
The Derivative Answer Trap — How Modern NBME Questions Work
The NBME has largely abandoned "what is the diagnosis?" in favor of "what is the mechanism?" or "what would you find?" or "what is the most likely explanation?" This means:
- Instead of "Hashimoto's thyroiditis" → answer is "autoimmune destruction of thyroid follicular cells"
- Instead of "obstructive sleep apnea" → answer is "hypercapnia due to daytime hypoventilation"
- Instead of "AIS" → answer is "XY karyotype with phenotypically female presentation"
- Instead of "Kartagener's" → answer is "impaired sperm motility due to ciliary dynein arm defect"
The Mechanism Chain Rule
For any fact you study, trace it: What causes it? What does it cause? What else causes the same thing? What else does it cause? This 4-directional thinking produces 4x the exam return on every study minute. Example: Prolactin → inhibits GnRH → suppresses FSH/LH → amenorrhea. Causes of ↑ prolactin: antipsychotics (D2 block), TRH (hypothyroidism), metoclopramide, opioids, CKD, MEN1, prolactinoma.
Autoimmune Disease Clustering — Clinical Pattern
| First Autoimmune Disease | High-Association Second Disease |
|---|---|
| Celiac disease | Hashimoto's thyroiditis, Type 1 DM, dermatitis herpetiformis |
| SLE | Antiphospholipid syndrome, Sjogren's, ITP, AIHA |
| Type 1 DM | Hashimoto's, Addison's, celiac, vitiligo |
| Rheumatoid Arthritis | Sjogren's, vasculitis, Felty syndrome, ILD |
| Hashimoto's | Type 1 DM, celiac, SLE, vitiligo, Addison's |
EP446
Divine Patch — Updates & Corrections from Past Episodes
- Guideline updates matter: USMLE reflects current clinical practice. Screening ages and pharmacologic preferences shift over time. When in doubt, follow the most recent ACS/USPSTF guidance — but note that NBME generally favors ACS over USPSTF for breast cancer screening (start at 40, not 50)
- Foundation quality determines ceiling: Students who did poorly on Step 1 overwhelmingly do poorly on Step 2 and Step 3. The solution is not memorizing more facts — it is building better pathophysiologic understanding. Poor foundation = same ceiling on all exams
- Normal saline is almost always right for fluids: NBME rarely discriminates between NS and LR on clinical questions. The exception: large-volume paracentesis → follow with IV albumin (to restore oncotic pressure and prevent paracentesis-induced circulatory dysfunction)
- Peripheral IVs vs central lines: Peripheral large-bore IVs have higher flow rates than central lines in acute resuscitation. Central lines are for long-term access (antibiotics, TPN, chemo), not necessarily fastest resuscitation
- Cancer + bone → bisphosphonates + radiation: Bone metastases cause pain and fracture risk. Radiation is first-line for pain relief. Bisphosphonates reduce skeletal events. Key side effect: osteonecrosis of the jaw (especially with IV zoledronic acid)
- Trastuzumab cardiomyopathy is reversible: Unlike doxorubicin (anthracycline → irreversible dilated cardiomyopathy via free radical damage), trastuzumab-induced cardiomyopathy reverses when drug is stopped. This distinction is tested
Cardiotoxicity Comparison — Key Drug Pairs
| Drug | Mechanism of Cardiotoxicity | Reversibility | Prevention |
|---|---|---|---|
| Doxorubicin / Daunorubicin (anthracyclines) | Free radical damage via Fenton reaction → cardiac myocyte death | Irreversible dilated cardiomyopathy | Dexrazoxane (iron chelator = Fenton reaction inhibitor); get baseline echo |
| Trastuzumab (Herceptin) | HER2 blockade disrupts cardiac repair mechanism | Reversible — cardiomyopathy resolves on stopping | Baseline echo; monitor EF; avoid concurrent doxorubicin |
| Cyclophosphamide | Hemorrhagic myocarditis at high doses | Usually reversible | Mesna (prevents hemorrhagic cystitis, not cardiac toxicity) |
Before Starting Chemotherapy — Cardiac Workup
If a patient is about to start doxorubicin or any anthracycline, the next best step is echocardiogram to establish baseline ejection fraction. If EF drops during treatment, the drug must be dose-adjusted or discontinued. This is a classic NBME "next best step" question setup.
High-Yield Integrated Cases
EP630 illustrates exactly how NBME questions integrate multiple systems in one vignette. Case 1 chains celiac disease → Hashimoto's → hypothyroidism → TRH → prolactin → GnRH suppression → amenorrhea → bradycardia — all from one set of symptoms. Case 2 chains obesity hypoventilation → hypercapnia → restrictive physiology → hypoxia → polycythemia → hypercoagulability → high-output heart failure.
EP630
2 High-Yield Integrated Cases (Step 1–3)
- Case 1 — Celiac + Hypothyroidism chain: Celiac disease (autoimmune) → associated with Hashimoto's thyroiditis (second autoimmune disease) → ↓ T3/T4 → ↓ negative feedback → ↑ TRH (powerful prolactin stimulator) → ↑ prolactin → GnRH suppression → irregular menses. Plus: ↓ T3 → fewer beta-1 receptors on cardiac myocytes → bradycardia (heart rate 49). Thyroglobulin = C-peptide of the thyroid gland (low when thyroid not making hormone)
- Case 2 — Obesity hypoventilation syndrome: Daytime hypercapnia (respiratory acidosis) + somnolence + daytime hypoventilation + HTN. Kidney compensates: ↑ bicarb (metabolic alkalosis). AA gradient = normal (lung parenchyma intact — it's a ventilation problem, not a diffusion problem). DLCO = normal. Lung volumes ↓ (restrictive — abdominal girth compresses thoracic cavity)
- OHS hypoxia cascade: Restricted ventilation → ↓ PAO2 → ↓ PaO2 → ↓ SaO2 → ↓ oxygen content → EPO rises → polycythemia → ↑ blood viscosity → stasis → hypercoagulability (Virchow's triad). Long-term: chronic high cardiac output → high-output heart failure
- OHS management: Polysomnogram to confirm. CPAP (= non-invasive positive pressure ventilation). Weight loss. Pharmacotherapy on NBME = tirzepatide (GLP-1/GIP dual agonist). Distinguish OHS from OSA by presence of daytime hypercapnia
- Thyroglobulin as thyroid marker: Released with T3/T4 from follicular cells. Acts like C-peptide — if thyroid is not making hormone (Hashimoto's), thyroglobulin is low. If thyroid is overactive or has a tumor, thyroglobulin is elevated. Used to monitor thyroid cancer after thyroidectomy
- Pregnancy restrictive physiology: Gravid uterus in third trimester compresses abdominal cavity → thoracic cavity squished → restrictive lung disease pattern. Normal DLCO. Normal AA gradient. Reduced lung volumes. Dyspnea of pregnancy has a physiologic basis
Case 1 — Complete Mechanism Chain
| Step | Event | Exam Answer Type |
|---|---|---|
| 1 | Celiac disease = autoimmune → high risk for second autoimmune disease | PMH is a clue |
| 2 | Hashimoto's thyroiditis → anti-TPO antibodies destroy follicular cells | Mechanism = autoimmune cell destruction |
| 3 | ↓ T3/T4 → ↓ negative feedback on hypothalamus/pituitary | Pathway |
| 4 | ↑ TRH → stimulates prolactin from anterior pituitary | TRH = prolactin stimulator (tested) |
| 5 | ↑ Prolactin → inhibits GnRH → ↓ FSH/LH → irregular/absent menses | HPG axis suppression |
| 6 | ↓ T3 → fewer beta-1 receptors on cardiac myocytes → bradycardia | Receptor density mechanism |
| 7 | Thyroglobulin low (not producing hormone) = surrogate marker | C-peptide analogy |
OHS vs OSA — Key Differentiator
Both obstructive sleep apnea (OSA) and obesity hypoventilation syndrome (OHS) cause daytime sleepiness. The distinguishing feature: OHS has daytime hypercapnia (elevated CO2 during the day), not just during sleep. OSA without OHS = normal daytime CO2. OHS = daytime CO2 retention. Both diagnosed with polysomnogram. Both treated with CPAP/BiPAP.
Lung Volume / Gas Exchange Framework
| Disease Type | Lung Volumes | DLCO | AA Gradient | Example |
|---|---|---|---|---|
| Obstructive (COPD, asthma) | ↑ RV, ↑ TLC (air trapping) | ↓ (emphysema) or Normal (asthma) | Normal or slightly ↑ | Emphysema: ↓ DLCO from alveolar destruction |
| Restrictive — intrinsic (ILD, fibrosis) | ↓ all volumes | ↓ (diffusion problem) | ↑ (V/Q mismatch) | IPF, asbestosis, sarcoidosis |
| Restrictive — extrinsic (OHS, kyphoscoliosis, pregnancy) | ↓ all volumes | Normal | Normal | OHS, neuromuscular disease, large pleural effusion |
| Marfan's (chest wall deformity) | ↓ all volumes | Normal | Normal | Pectus abnormalities restrict expansion |
Genetics & Basic Science Integration
Autosomal dominant disorders are tested by recognizing the phenotype and knowing two to three high-yield details per disease. EP455 surveys 20+ conditions. EP547 covers the basic science content that bleeds into Step 2/3 — not memorization fodder, but mechanism explanations for clinical presentations you will see on shelves and boards.
EP455
HY Autosomal Dominant Disorders
- Huntington's disease: CAG repeats on chromosome 4; Huntingtin gene. Choreiform movements + dementia + psychiatric symptoms in 40s. Caudate nucleus atrophy on imaging. Anticipation (worse in successive generations). AD inheritance — most trinucleotide repeat disorders are AD (exception: Friedreich's ataxia = AR)
- Marfan syndrome: Fibrillin gene (chromosome 15). Tall stature, arachnodactyly, lens dislocation upward/outward (vs homocystinuria = downward/inward). Mitral valve prolapse (myxomatous degeneration). Aortic dissection (cystic medial necrosis). Pneumothorax (tall thin body habitus → apical blebs). Circle of Willis aneurysms. Restrictive lung disease (pectus deformity)
- HOCM (hypertrophic obstructive cardiomyopathy): Beta-myosin heavy chain mutation. Sudden cardiac death in young athletes. Murmur louder with Valsalva/standing (↓ preload → LVOT obstruction worse); softer with squatting/fluids (↑ preload → obstruction relieved). Treat with beta-blockers (slows HR → more filling time → relieves obstruction)
- Tuberous sclerosis: TSC1 (hamartin) / TSC2 (tuberin) mutations, chromosome 16. Ash leaf spots (hypopigmented), adenoma sebaceum (facial angiofibromas), cardiac rhabdomyomas, renal angiomyolipomas, cortical tubers → infantile spasms (West syndrome → hypsarrhythmia on EEG → treat with ACTH). SEGA (subependymal giant cell astrocytoma)
- NF1 (von Recklinghausen's): Chromosome 17, neurofibromin gene. Café-au-lait spots (≥6, ≥1.5cm), Lisch nodules (hamartomas of iris), neurofibromas (posterior mediastinal masses), optic nerve gliomas, pheochromocytoma, meningiomas. NF2: bilateral acoustic neuromas, chromosome 22, merlin gene
- ADPKD: PKD1 (chromosome 16, most common) or PKD2 (chromosome 4). Hypertension + hematuria + flank pain + palpable kidneys. Berry aneurysms (circle of Willis). Mitral valve prolapse. Hepatic/pancreatic cysts. Colonic diverticula. Renal cell carcinoma risk. EPO-producing cysts → polycythemia
Autosomal Dominant Disorders — High-Yield Master Table
| Disorder | Gene / Chromosome | Key Mechanism | HY Clinical Pearl |
|---|---|---|---|
| Huntington's disease | HTT gene, chr 4 (CAG repeats) | Caudate atrophy; HD protein inhibits transcription | Anticipation; choreiform movements + dementia in 40s |
| Marfan syndrome | FBN1 (fibrillin), chr 15 | Defective extracellular matrix | Lens up/out; aortic dissection; cystic medial necrosis |
| HOCM | MYH7 (β-myosin heavy chain) | Asymmetric septal hypertrophy → LVOT obstruction | Murmur worsened by decreased preload |
| Tuberous sclerosis | TSC1/TSC2, chr 16 | mTOR pathway overactivation | Ash leaf spots + rhabdomyoma + infantile spasms |
| NF1 | NF1 (neurofibromin), chr 17 | RAS-GAP tumor suppressor loss | Café-au-lait + Lisch nodules + neurofibromas |
| NF2 | NF2 (merlin), chr 22 | Schwannoma tumor suppressor loss | Bilateral acoustic neuromas = pathognomonic |
| VHL | VHL, chr 3 | Ubiquitin ligase defect → HIF-1α not degraded | Cerebellar hemangioblastoma + clear cell RCC + pheochromocytoma |
| ADPKD | PKD1 (chr 16) or PKD2 (chr 4) | Abnormal ciliary signaling → cyst formation | Berry aneurysms; MVP; hepatic cysts; colonic diverticula |
| Familial hypercholesterolemia | LDLR (LDL receptor) | LDL not cleared → accumulates in circulation | MI in 20s–30s; tendon xanthomas; treat with statins + PCSK9 inhibitors |
| FAP | APC, chr 5 | β-catenin regulation failure → polyps | Hundreds of colon polyps; prophylactic colectomy; Gardner = soft tissue tumors + polyps |
| HHT (Osler-Weber-Rendu) | ENG/ACVRL1 | AVM formation → shunting | Telangiectasias + AVM + epistaxis + high-output CHF |
| Myotonic dystrophy | DMPK (protein kinase), CTG repeats | RNA toxic gain of function | Grip myotonia + frontal balding + cataracts + arrhythmias + testicular atrophy |
| Li-Fraumeni syndrome | TP53, chr 17 | p53 tumor suppressor loss | Many cancers (breast, brain, leukemia, adrenal, sarcoma) in young patients and families |
| Hereditary spherocytosis | SPTA1/ANK1 (spectrin/ankyrin) | RBC membrane defect → splenic destruction | Coombs-negative hemolytic anemia; ↑ MCHC; splenectomy curative |
| Von Willebrand disease | VWF gene | VWF deficiency → impaired platelet adhesion + ↓ factor VIII stability | ↑ bleeding time + ↑ aPTT; treat with desmopressin (releases VWF from Weibel-Palade bodies) |
| AIP (acute intermittent porphyria) | HMBS (porphobilinogen deaminase) | Heme synthesis block → PBG accumulation | Abdominal pain + neuropsychiatric + port-wine urine; precipitated by drugs/fasting; treat with heme/glucose |
Trinucleotide Repeat Disorders Summary
Most trinucleotide repeat disorders = autosomal dominant with anticipation (worsening over generations as repeat count increases). Key exceptions:
Friedreich's ataxia = autosomal recessive (GAA repeats, frataxin gene) — spinocerebellar degeneration + hypertrophic cardiomyopathy + diabetes.
Fragile X syndrome = X-linked dominant (CGG repeats, FMR1 gene) — most common inherited intellectual disability; macroorchidism; long face; autism features.
Huntington's = CAG (chr 4), AD. Myotonic dystrophy = CTG (DMPK), AD.
EP547
HY Basic Sciences for Step 2 & 3
- Red blood cell size adaptation (why microcytosis occurs): Hemoglobin concentration = mass/volume. When mass of hemoglobin decreases (iron deficiency, thalassemia, lead poisoning), the RBC reduces its volume proportionally to maintain hemoglobin concentration. This is the mechanistic explanation for microcytosis — a compensatory response, not a primary defect in cell size
- Macrocytosis mechanism (B12/folate): B12 or folate deficiency → impaired DNA synthesis → cells can't divide properly but continue to grow → cell size increases disproportionately to nuclear division → macrocytic RBCs. The bottleneck is at DNA replication (S phase), not at protein/membrane synthesis
- Anemia of chronic disease vs iron deficiency: ACD: hepcidin elevated → blocks GI iron absorption AND blocks iron release from macrophages → ferritin HIGH (iron stored in macrophages), TIBC LOW, transferrin saturation LOW. Iron deficiency: ferritin LOW, TIBC HIGH (hungry for iron), transferrin sat LOW. Both have low iron in circulation — the ferritin distinguishes them
- Alpha thalassemia diagnosis: Do NOT use hemoglobin electrophoresis for alpha thalassemia — it may be normal because all hemoglobins contain alpha chains, so proportions appear normal. Beta thalassemia = hemoglobin electrophoresis shows ↑ HbA2 (alpha2-delta2) and ↑ HbF. Alpha thalassemia = diagnose with DNA analysis or CBC patterns
- Lead poisoning mechanism of microcytic anemia: Lead inhibits ALA dehydratase (early step) and ferrochelatase (final step — combines iron + protoporphyrin → heme). Both blocks → ↓ heme production → microcytic anemia. Additional: peripheral neuropathy (demyelination) + abdominal pain + basophilic stippling on smear
- Hereditary spherocytosis vs AIHA: Both cause hemolytic anemia. HS = Coombs negative (not antibody-mediated). AIHA = Coombs positive (antibody-mediated). HS = ↑ MCHC (concentrated hemoglobin in smaller cell). Both = ↑ indirect bilirubin + ↑ LDH + ↓ haptoglobin. Treat HS with splenectomy. Treat AIHA with steroids
Anemia Classification and Key Labs
| Anemia Type | MCV | Reticulocyte Count | Ferritin | TIBC | Key Distinguisher |
|---|---|---|---|---|---|
| Iron deficiency | ↓ (microcytic) | ↓ | ↓↓ | ↑↑ | Most common anemia; GI blood loss in adults |
| Beta thalassemia minor | ↓ (microcytic) | Normal–↑ | Normal–↑ | Normal | HbA2 ↑ on electrophoresis; Mediterranean ancestry |
| Anemia of chronic disease | Normal (normocytic) or ↓ | ↓ | ↑ (iron trapped in macrophages) | ↓ | Hepcidin elevated; chronic inflammatory disease; treat underlying cause |
| B12 / Folate deficiency | ↑ (macrocytic) | ↓ | Normal | Normal | B12: ↑ methylmalonic acid + ↑ homocysteine. Folate: only ↑ homocysteine. B12 deficiency = peripheral neuropathy + subacute combined degeneration |
| Hereditary spherocytosis | Normal or ↓ | ↑↑ | Normal–↑ | Normal | MCHC ↑; Coombs negative; osmotic fragility ↑; splenectomy curative |
| AIHA (autoimmune hemolytic) | Normal or ↑ (reticulocytosis) | ↑↑ | Normal–↑ | Normal | Coombs positive; warm (IgG) or cold (IgM); treat with steroids (warm type) |
| Aplastic anemia | Normal | ↓↓↓ | Normal–↑ | Normal | Pancytopenia; bone marrow failure; causes: idiopathic, chloramphenicol, benzene, parvovirus |
The Concentration Model for Microcytosis
Hemoglobin concentration = mass/volume (C = M/V). When hemoglobin mass decreases (iron deficiency, thalassemia, lead poisoning), the body compensates by decreasing cell volume to maintain concentration near homeostasis. This is why microcytosis is a compensation, not the primary defect. The MCHC may stay normal or near-normal until compensation fails.
Special Populations & High-Yield Concepts
EP379 covers the high-yield elderly vignette patterns — which conditions are more common in older adults, which drugs should be avoided, and how presentations differ from younger patients. EP504 covers the super high-yield USMLE concepts that cut across all systems and appear frequently on shelves and boards.
EP379
HY Elderly Vignettes — A Series
- Delirium vs dementia: Delirium = acute, fluctuating, inattention as core feature; often reversible (medications, infection, metabolic, pain). Dementia = insidious onset, progressive, preserved attention early. Dementia is a risk factor for delirium. Hospital-acquired delirium = avoid anticholinergics (diphenhydramine, oxybutynin), opioids, benzodiazepines
- Falls in the elderly: Multifactorial. Most common cause on NBME = postural hypotension (orthostatic) — especially with antihypertensives, alpha-blockers, diuretics. Check orthostatics first. Other causes: vision impairment, medications (benzodiazepines, antihistamines), peripheral neuropathy, vestibular dysfunction
- Urinary incontinence types: Stress (coughing/sneezing → leak; weak sphincter) = Kegel exercises → pessary → surgical sling. Urge (overactive bladder; can't hold) = bladder training → oxybutynin (avoid in elderly → anticholinergic). Overflow (retention → overflow; enlarged prostate or diabetic neuropathy) = catheter + treat cause. Functional (can't get to bathroom; dementia) = scheduled toileting
- Polypharmacy and Beers Criteria: Drugs to avoid in elderly: NSAIDs (GI bleed, renal failure), benzodiazepines (fall risk, cognitive impairment), diphenhydramine (anticholinergic delirium), meperidine (normeperidine toxicity → seizures), first-generation antipsychotics (QT prolongation, extrapyramidal), metoclopramide (tardive dyskinesia, parkinsonism)
- Pseudogout in elderly: Most common crystal arthropathy in elderly patients. CPPD crystals. Knee joint most common. Chondrocalcinosis on X-ray. Associated with hyperparathyroidism, hemochromatosis, hypomagnesemia. Treat with NSAIDs or intraarticular steroids
- Giant cell arteritis: Disease of elderly (>50, mean onset 70s). Temporal headache + jaw claudication + scalp tenderness + visual changes (anterior ischemic optic neuropathy → permanent blindness if untreated). ESR >50. Start prednisone immediately — do not wait for biopsy result. Temporal artery biopsy confirms (skip lesions — can be false negative)
Delirium — Reversible Causes (I WATCH DEATH Mnemonic)
| Letter | Category | Examples |
|---|---|---|
| I | Infections | UTI, pneumonia, meningitis |
| W | Withdrawal | Alcohol, benzodiazepines, opioids |
| A | Acute metabolic | Hypo/hypernatremia, hypo/hyperglycemia, liver/renal failure |
| T | Trauma | Head trauma, surgery (post-op delirium) |
| C | CNS pathology | Stroke, seizure (post-ictal), meningitis |
| H | Hypoxia | PE, COPD exacerbation, heart failure |
| D | Deficiencies | B12, thiamine (Wernicke's), folate |
| E | Endocrine | Thyroid, adrenal, glucose disorders |
| A | Acute vascular | Stroke, MI, arrhythmia |
| T | Toxins | Medications (anticholinergics, opioids), alcohol intoxication |
| H | Heavy metals | Lead, arsenic, mercury |
Drugs Most Dangerous in Elderly — Quick Reference
Benzodiazepines: Fall risk, cognitive impairment, respiratory depression. Avoid even short-acting ones.
Diphenhydramine (Benadryl): Highly anticholinergic → delirium, urinary retention, constipation, dry mouth, blurred vision.
NSAIDs: GI bleeding (no COX-1 protection) + acute kidney injury + fluid retention → heart failure exacerbation.
Meperidine: Normeperidine metabolite → seizures (not reversed by naloxone). Use morphine or hydromorphone instead.
Metoclopramide: Long-term → tardive dyskinesia (irreversible), parkinsonism. Avoid in elderly.
EP504
Super HY Discussion of USMLE Concepts
- Toxoplasma in HIV: Ring-enhancing lesions in brain of HIV patient → treat empirically for toxo (pyrimethamine + sulfadiazine + leucovorin). If no response in 2 weeks → biopsy (primary CNS lymphoma). Prophylaxis = TMP-SMX (same drug as PCP prophylaxis). Toxo in pregnancy → spiramycin (not pyrimethamine/sulfadiazine — teratogenic folate inhibitors)
- EBV-associated malignancies: Primary CNS lymphoma (HIV patient), Burkitt's lymphoma (t(8;14), starry-sky pattern, African jaw mass), nasopharyngeal carcinoma, Hodgkin's lymphoma (Reed-Sternberg cells), post-transplant lymphoproliferative disorder (PTLD)
- Alpha-1 antitrypsin deficiency + cirrhosis: Unfolded A1AT accumulates in hepatocytes → hepatocyte apoptosis → cirrhosis. Simultaneously: no A1AT to inhibit elastase → elastase destroys alveoli → emphysema. Smoking dramatically accelerates lung disease. No smoking = relatively normal life expectancy. With smoking = severe COPD in 40s + early cirrhosis
- Esophageal varices management: Acute: 2 large-bore IVs + NS + blood if needed + sclerotherapy/banding + IV octreotide + PPI. Chronic prophylaxis: propranolol/nadolol (splanchnic vasoconstriction). SBP prophylaxis (fluoroquinolone or ceftriaxone) required in all cirrhotics with variceal bleeding
- Actinic keratosis: Pre-malignant skin lesion; sandpaper-like texture; biggest risk factor = sun exposure; most resolve; subset → squamous cell carcinoma. Treat with 5-fluorouracil cream or imiquimod. Keratin pearls can be seen in both actinic keratosis AND squamous cell carcinoma
- Parvo B19 in sickle cell: Parvo B19 infects erythroid precursors → aplastic crisis (reticulocyte count falls). In hemoglobinopathy patients (sickle cell, thalassemia), a LOW reticulocyte count = parvo B19 until proven otherwise. These patients normally have HIGH reticulocyte counts — a paradoxically low retic count is the clue
HIV Opportunistic Infections — CD4 Count and First-Line Tx
| CD4 Threshold | OI / Condition | Prophylaxis | Treatment |
|---|---|---|---|
| <200 cells/µL | PCP (Pneumocystis jirovecii pneumonia) | TMP-SMX | TMP-SMX + steroids if PaO2 <70 or A-a gradient >35 |
| <200 cells/µL | Toxoplasmosis (ring-enhancing CNS) | TMP-SMX (same as PCP prophylaxis) | Pyrimethamine + sulfadiazine + leucovorin |
| <100 cells/µL | Cryptococcal meningitis | Fluconazole | Amphotericin B + flucytosine → step down fluconazole |
| <50 cells/µL | MAC (Mycobacterium avium complex) | Azithromycin | Clarithromycin + ethambutol ± rifabutin |
| <50 cells/µL | CMV retinitis | Ganciclovir (or valganciclovir) | Ganciclovir IV or valganciclovir oral |
| Any CD4 | TB reactivation | IGRA/PPD screen → if positive, INH × 9 months | RIPE × 6 months (longer if MDR) |
Candida on Wet Prep — What to Look For
Vaginal candidiasis: pseudohyphae (branching filamentous structures) + budding yeast cells on wet prep. Vaginal pH ≤4.5 (acidic, unlike BV which is >4.5). Treat with topical azole (clotrimazole) or oral fluconazole. In HIV patients, candida can cause esophagitis (odynophagia — pain with swallowing, not just heartburn). Esophageal candidiasis → systemic fluconazole (not topical).