Foundations of Medicine: Unit 3 Summary (Weeks 1.3.1–1.3.6)

High-yield practice flashcards covering Weeks 1.3.1 to 1.3.6 of Foundations of Medicine Unit 3, including immunology, infectious diseases, pharmacology, dermatology, rheumatology, endocrinology, temperature regulation, and palliative care.

What are the two major lymphatic ducts that return excess interstitial fluid to the venous system, and which anatomical regions do they drain?

The thoracic duct empties into the left subclavian vein and drains approximately $\frac{3}{4}$ of the body, while the right lymphatic duct empties into the right subclavian vein and drains the remaining smaller territory ($\sim \frac{1}{4}$).

How do corticosteroids fundamentally differ from biologic therapies in their mechanism of treating autoimmune diseases?

Corticosteroids exert a broad, non-specific dampening effect across all cell types leading to systemic side effects, whereas biologic agents specifically target a single step, receptor, or cytokine in the inflammatory pathway.

What are the only three recognized contraindications to vaccination in Canada?

(1) Anaphylaxis to a previous dose of any vaccine, (2) administration of a live vaccine during pregnancy, and (3) administration of a live vaccine in the setting of severe immunosuppression or known immunodeficiency.

What is the only vaccine adjuvant currently marketed in Canada, and what is its role?

Aluminum salts; they function to enhance the immune response to protein antigens.

How do first-order and zero-order drug elimination differ in their clearance kinetics and reliance on half-life?

First-order elimination removes a constant fraction of the drug per unit time, meaning half-life applies. Zero-order elimination removes a constant absolute amount per unit time, meaning half-life does not apply.

What are the five layers of the epidermis from deep to superficial, and which layer houses Langerhans cells?

Stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (thick skin only), and stratum corneum. Langerhans cells reside in the stratum spinosum.

What morphological characteristics distinguish a macule from a patch, and a papule from a plaque?

A macule is flat and $< 1\,\text{cm}$, whereas a patch is flat and $> 1\,\text{cm}$. A papule is an elevated solid lesion $< 1\,\text{cm}$, whereas a plaque is an elevated solid lesion $> 1\,\text{cm}$.

What is the molecular pathophysiology underlying Staphylococcal Scalded Skin Syndrome (SSSS)?

Staphylococcus aureus exotoxins A and B specifically cleave desmosomes in the epidermis, resulting in loss of cell-cell adhesion and widespread superficial epidermal peeling in children under $5$ years of age.

What is a Majocchi granuloma, and what clinical error causes it to form?

A deep, granulomatous fungal folliculitis that occurs when a superficial dermatophyte infection is inappropriately treated with topical corticosteroids, which suppresses local immunity and drives fungal organisms down into the hair follicles.

How do the clinical presentations and viral etiologies of Roseola infantum and Erythema infectiosum differ?

Roseola (HHV-6) features high fever lasting $3\text{--}5\,\text{days}$ followed by a rose-pink blanching rash appearing immediately after fever resolution. Erythema infectiosum (parvovirus B19) presents with a 'slapped cheek' facial rash, carrying a risk of aplastic crisis in patients with underlying red blood cell disorders.

What is the standard first-line treatment for scabies, and why must the dose be repeated in $7\,\text{days}$?

Permethrin $5\%$ cream applied from neck to soles for $12\,\text{hours}$. It is repeated in $7\,\text{days}$ because the initial dose does not eradicate unhatched eggs; repeating breaks the life cycle once newly hatched eggs emerge.

Why is intramuscular epinephrine the definitive first-line medication in anaphylaxis, whereas antihistamines are considered only adjuncts?

Epinephrine acts on $\alpha_1$ receptors (vasoconstriction reversing hypotension and edema), $\beta_1$ receptors (increasing heart rate and contractility), and $\beta_2$ receptors (bronchodilation). Antihistamines merely block $\text{H}_1$ receptors and cannot reverse airway constriction or circulatory collapse.

What are the three primary physiological outcomes mediated by the complement system?

(1) Assembly of the membrane attack complex (MAC) to lyse microbial membranes, (2) opsonization to mark pathogens for phagocytosis, and (3) induction of inflammation to recruit leukocytes.

Which conditions are classically identified by the presence of Reed-Sternberg cells, hypersegmented neutrophils, and Auer rods on a peripheral blood smear?

Reed-Sternberg cells indicate classic Hodgkin lymphoma, hypersegmented neutrophils ($\ge 5$ lobes) indicate megaloblastic anemia, and Auer rods indicate acute myeloid leukemia (AML).

What chemical signalling cascade mediates an infection-driven rise in the hypothalamic temperature set point?

Exogenous pyrogens trigger macrophages/monocytes to secrete endogenous pyrogens (IL-1, IL-6, TNF, IFN), which stimulate hypothalamic endothelial synthesis of prostaglandin $\text{PGE}_2$ via arachidonic acid, driving cAMP production to elevate the thermoregulatory set point.

At what core body temperature range does hypothermia become 'moderate', and what physiological defence is lost?

Between $28\text{--}32\,^\circ\text{C}$; the shivering reflex ceases, depriving the body of its primary mechanism of active heat production.

What are the three classic systemic 'B symptoms' that warrant screening for underlying lymphoma or malignancy?

Fever, drenching night sweats (soaking clothes or sheets), and unexplained weight loss exceeding $10\%$ of baseline body weight over $6\,\text{months}$.

What is the cell surface target and lifecycle sequence of HIV within host immune cells?

HIV binds the $\text{CD4}$ molecule on helper T lymphocytes, enters the cell, undergoes reverse transcription (RNA into DNA), integrates viral DNA into the host genome, and replicates, eventually depleting host $\text{CD4}$ cells.

What diagnostic thresholds define a clinical diagnosis of AIDS in an HIV-positive patient?

A $\text{CD4}$ T-cell count falling below $200\,\text{cells}/\mu\text{L}$ OR the development of an AIDS-defining condition (such as Pneumocystis jirovecii pneumonia or Kaposi sarcoma).

What is the standard initial 6-month antimycobacterial regimen for drug-susceptible active pulmonary tuberculosis?

RIPE therapy (Rifampin, Isoniazid, Pyrazinamide, and Ethambutol) for the initial $2\,\text{months}$, followed by $4\,\text{months}$ of continuation therapy with Rifampin and Isoniazid ($6\,\text{months}$ total).

What diagnostic evaluations are mandatory in a full septic workup for a febrile infant under $21\,\text{days}$ of age?

Complete blood count (CBC) with differential, C-reactive protein (CRP), blood cultures, catheterized urinalysis with urine culture, and lumbar puncture for CSF cell count, protein, glucose, culture, and HSV PCR.

Which major bacterial genus exhibits intrinsic resistance across all generations of cephalosporin antibiotics?

Enterococcus species are intrinsically resistant to all cephalosporins.

Why is ciprofloxacin clinically unique in Canada, and why is metronidazole strictly ineffective against aerobic bacteria?

Ciprofloxacin is the only oral anti-pseudomonal antibiotic available in Canada. Metronidazole requires reduction by anaerobic nitroreductases to generate cytotoxic free radicals, rendering it ineffective in aerobic environments.

Which physical examination findings distinguish orbital cellulitis from preseptal (periorbital) cellulitis?

Orbital cellulitis presents with signs of retroseptal involvement: proptosis, ophthalmoplegia, pain with extraocular movements, decreased visual acuity, and chemosis.

Which topical antimicrobial agents are strictly contraindicated in otitis externa if the tympanic membrane is perforated?

Aminoglycosides (neomycin, tobramycin, gentamicin), polymyxin B, and acidic solutions must be avoided due to ototoxicity; topical fluoroquinolones (such as ciprofloxacin) are safe.

How do vaginal pH, wet mount microscopy, and the KOH whiff test differentiate Bacterial Vaginosis (BV) from Vulvovaginal Candidiasis (VVC)?

BV demonstrates a $\text{pH} > 4.5$, clue cells on wet mount, and a positive fishy amine whiff test. VVC maintains a normal acidic $\text{pH} < 4.5$, demonstrates fungal hyphae/buds on microscopy, and yields a negative whiff test.

What typical synovial fluid profile and autoantibody combination characterize Rheumatoid Arthritis?

Synovial fluid is translucent yellow with a WBC count of $15{,}000\text{--}25{,}000\,\text{cells}/\text{mm}^3$ ($> 50\%$ PMNs) and depressed complement levels. Serum labs typically show positive rheumatoid factor (RF) and high-specificity anti-cyclic citrullinated peptide (anti-CCP) antibodies.

What is the clinical role of an Antinuclear Antibody (ANA) screen versus an anti-dsDNA test in suspected SLE?

ANA is highly sensitive ($> 95\%$) but non-specific, serving as an entry criterion. Anti-dsDNA is highly specific for SLE, correlates with disease activity and lupus nephritis, but its absence does not exclude disease.

By what cellular mechanisms do systemic corticosteroids promote osteoporosis?

Corticosteroids increase renal calcium excretion, reduce intestinal calcium absorption, prolong osteoclast lifespan, and accelerate osteoblast apoptosis.

What is the immunological mechanism and expected timing of Heparin-Induced Thrombocytopenia (HIT) Type 2?

HIT Type 2 is mediated by antibodies directed against platelet factor 4 (PF4)-heparin complexes that activate platelets, typically presenting $5\text{--}10\,\text{days}$ after heparin exposure with thrombocytopenia and paradoxical venous/arterial thrombosis.

What are the diagnostic criteria for Kawasaki disease, and what is its most critical cardiovascular complication?

Fever for $\ge 5\,\text{days}$ plus at least $4$ of $5$ signs: bilateral non-purulent conjunctivitis, oral changes (strawberry tongue, cracked lips), peripheral extremity changes (erythema, edema, desquamation), polymorphous rash, and cervical lymphadenopathy ($> 1.5\,\text{cm}$). The critical complication is coronary artery aneurysm.

What clinical hallmark distinguishes IgA vasculitis (Henoch-Schönlein purpura) from idiopathic thrombocytopenic purpura (ITP)?

IgA vasculitis features palpable purpura (most prominent on dependent lower limbs and buttocks) occurring in the presence of completely normal platelet counts and normal coagulation parameters.

How are Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) categorized by body surface area (BSA) of epidermal detachment?

SJS involves $< 10\%$ BSA detachment; SJS/TEN overlap involves $10\text{--}30\%$ BSA detachment; and TEN involves $> 30\%$ BSA detachment.

How do serum TSH and free T4 levels differentiate primary from central hypothyroidism?

Primary hypothyroidism shows elevated TSH alongside low free T4 (end-organ failure). Central hypothyroidism shows low or inappropriately normal TSH alongside low free T4 (pituitary or hypothalamic axis failure).

How does radioactive iodine uptake differentiate Graves' disease from subacute thyroiditis in a patient with thyrotoxicosis?

Graves' disease exhibits high, homogeneous uptake due to active hormone synthesis driven by TSH-receptor antibodies. Subacute thyroiditis exhibits low/absent uptake because follicular destruction releases pre-stored hormone without active synthesis.

What is the initial investigation when an adult presents with a thyroid nodule, and what does a suppressed result imply?

Serum TSH. A suppressed (low) TSH indicates an autonomously functioning ('hot') nodule causing hyperthyroidism; such nodules are almost never malignant and warrant a thyroid scan rather than biopsy.

What dynamic physical exam feature identifies a thyroglossal duct cyst?

A thyroglossal duct cyst is located in the anterior neck midline and elevates/moves upward with swallowing and tongue protrusion due to attachment to the tongue base along the descent tract.

What diagnostic categories make up the DIMS mnemonic used in investigating the underlying cause of delirium?

Drugs (prescriptions, toxicity, withdrawals), Infections (systemic, CNS, urinary, pulmonary), Metabolic (electrolytes, organ failure, endocrine, nutrition), and Structural/Systems (CNS injury, stroke, retention, cardiovascular low-output).

What is an Alberta Personal Directive, and under what specific condition does it come into effect?

A legal advance care planning document naming a designated alternate decision-maker ('agent') and personal instructions; it takes effect only when the individual loses the capacity to make their own personal or healthcare decisions.

What clinical objectives define the three overarching Alberta Goals of Care Designation categories: R, M, and C?

R (Resuscitative): Curative or life-prolonging focus including ICU admission (R1 includes CPR; R2/R3 exclude CPR). M (Medical): Active hospital medical/surgical treatment without ICU care. C (Comfort): Care focused entirely on symptom control and quality of life when death is expected, excluding curative life-prolonging interventions.