ABAI common mistakes
ABAI · Understand the traps and how to avoid them.
On a question describing a patient with recurrent angioedema and normal C4 during attacks, the candidate selects 'hereditary angioedema' because that is the only angioedema diagnosis they have memorised.
Pattern-matching on the single keyword 'angioedema' without checking the complement results; HAE with normal C4 (or acquired forms) and ACE-inhibitor angioedema are less rehearsed, so the familiar label wins.
What to do instead
Before committing, write the one discriminating datum next to each candidate diagnosis (C4 during attack, C1-INH level/function, medication history, family history) and eliminate any option contradicted by a stated result.
The candidate treats a positive skin-prick test or elevated specific IgE as proof of clinical allergy and chooses 'avoid the food/drug' even when the history shows tolerance on ingestion.
Sensitisation and allergy are conflated because both are reported as 'positive'; the stem's clinical history is read as background rather than as the decisive evidence.
What to do instead
For every positive test, ask 'does the history confirm symptoms on exposure?' If the patient tolerates the agent, the test is evidence of sensitisation only; choose the option consistent with tolerance, not avoidance.
On an anaphylaxis item the candidate picks epinephrine as the answer only when hypotension is described, and chooses an antihistamine or corticosteroid when the stem lists only urticaria plus one non-hypotensive system (e.g. persistent cough or vomiting).
The candidate holds an implicit 'shock = epinephrine' rule and under-weights that anaphylaxis is defined by involvement of two organ systems after exposure, not by blood pressure alone.
What to do instead
Apply the two-system rule explicitly: skin/mucosa plus respiratory or GI or cardiovascular, or isolated hypotension after a known allergen, means anaphylaxis and first-line intramuscular epinephrine.
The candidate answers a chronic urticaria question by ordering an extensive allergy work-up (food-specific IgE panels, autologous serum testing, extensive laboratory screens) rather than choosing symptomatic second-generation antihistamine therapy with escalation.
The candidate assumes every urticaria question is an allergy-identification question; acute and chronic urticaria have different diagnostic yields, and the low-yield work-up is a familiar distractor set.
What to do instead
Segment by duration: acute urticaria is usually triggered and needs no routine testing; chronic spontaneous urticaria is diagnosed clinically and treated by antihistamine uptitration before advanced agents or limited targeted testing.
The candidate selects a beta-lactam for a patient with a documented penicillin reaction without checking the reaction phenotype, or conversely avoids all beta-lactams in a patient whose reaction was a benign, non-immunologic one (e.g. isolated GI upset).
The label 'penicillin allergy' is treated as a binary contraindication; the candidate does not classify reactions by timing and mechanism, which determines cross-reactivity risk and whether a direct challenge is appropriate.
What to do instead
Classify first (immediate vs delayed, severe cutaneous adverse reaction vs benign rash vs intolerance), then decide: severe delayed reactions require strict avoidance, while low-risk histories may permit direct oral challenge or a graded approach.
When asked which primary immunodeficiency to suspect, the candidate chooses the option matching the single most memorable feature (e.g. 'low IgG' for any antibody complaint) without checking the pattern of infections, age of onset, and associated findings in the stem.
Immunodeficiency categories are memorised as isolated facts rather than as recognisable patterns (antibody, combined, phagocyte, complement), so a single lab value overrides the clinical picture.
What to do instead
Build a one-line pattern card per category (e.g. recurrent sinopulmonary infections with encapsulated organisms and low immunoglobulins = antibody defect; recurrent Neisseria = complement; deep pyogenic abscesses and poor wound healing = phagocyte) and match the whole stem to the card.
The candidate interprets a normal total immunoglobulin level or a normal lymphocyte count as excluding a clinically significant immunodeficiency and selects 'no further testing needed'.
Screening tests are mistaken for diagnostic tests; the candidate does not account for specific antibody deficiency, functional defects, or age-specific reference ranges.
What to do instead
Treat normal screening values as non-excluding when the history is suggestive; choose the option that proceeds to functional assessment (e.g. vaccine response, specific antibody titres, complement function) rather than reassurance.
The candidate answers a question about live vaccines or immunoglobulin replacement in a patient with antibody deficiency by applying the general rule for the wrong intervention, e.g. withholding all vaccines from a patient on immunoglobulin, or giving a live vaccine to a severely immunocompromised patient.
The candidate recalls the rule but not the exception or the reason behind it (passive antibody blunts response; live vaccines risk dissemination in severe cellular defects), so rules get swapped under time pressure.
What to do instead
State the mechanism before choosing: live vaccines are contraindicated in severe cellular immunodeficiency, while in antibody deficiency killed vaccines are generally safe and timing relative to immunoglobulin replacement governs efficacy, not safety.
The candidate selects a complement deficiency diagnosis on the basis of a low CH50 alone, without checking whether individual component levels or the clinical syndrome (e.g. angioedema vs recurrent Neisseria infection vs haemolytic anaemia) fit.
The screening-versus-specific distinction is blurred, and the candidate does not link complement pathway (classical, alternative, terminal, regulatory) to the characteristic clinical presentation.
What to do instead
Map pathway to syndrome before choosing: terminal component defects to recurrent Neisseria, C1 inhibitor to angioedema, regulatory protein defects to atypical haemolytic uraemic syndrome, and use the specific assay named in the stem to confirm.
The candidate answers an item about a drug reaction or transfusion reaction by naming the mechanism they associate with the drug class rather than the mechanism supported by the timing and features in the stem (e.g. calling a delayed maculopapular eruption 'type I immediate hypersensitivity').
The Gell-Coombs types are memorised as labels attached to examples, and the candidate does not re-derive the type from the described onset, cell involvement, and laboratory findings.
What to do instead
For each option, test the timeline and the effector described in the stem (minutes with mast cells, hours to days with antibody and complement, days with T cells); eliminate any mechanism whose timing or cell type is contradicted by the stem.