NRE Step 1 Community Medicine MCQs

Answer: 75%. PPV = true positives ÷ all test positives = 90 ÷ (90 + 30) = 75%. For comparison: sensitivity = 90 ÷ 100 = 90%; specificity = 170 ÷ 200 = 85%; NPV = 170 ÷ 180 ≈ 94%. Unlike sensitivity and specificity, predictive values change with disease prevalence.

• 60%: 60% does not come from this table; PPV is 90 ÷ (90 + 30) = 75%.
• 85%: 85% is the specificity: 170 ÷ (170 + 30).
• 90%: 90% is the sensitivity: 90 ÷ (90 + 10).

Answer: Ethics approval + informed consent. Research involving humans requires independent ethics committee (IRB/ERC) approval and informed consent that is voluntary, given by a competent person with full information on risks, benefits and the right to withdraw — principles of the Declaration of Helsinki. Consent from family alone is not sufficient for a competent adult. Vulnerable groups need extra safeguards.

• Ethics approval + family head consent: A competent adult must consent personally.
• Informed consent alone: Ethics committee review is also required.
• Sponsor consent + payment: Sponsor agreement and payment do not protect participants; ethics approval and informed consent do.

Answer: Routine surveillance (DHIS2). Health information sources: census (every ~10 years, population denominators), vital registration (births, deaths, causes — CRVS), routine facility data (HMIS/DHIS — service use, notifiable diseases), disease surveillance (Integrated Disease Surveillance and Response — immediate and weekly reporting with alert/epidemic thresholds), population surveys (Demographic and Health Survey, MICS, PSLM — coverage, fertility, nutrition every few years), disease registries (cancer). Surveillance types: passive (routine reporting — cheap, under-reports), active (staff search for cases, e.g. AFP), sentinel (selected sites), syndromic (symptom groups). Good HMIS data are complete, timely, accurate and used for decisions.

• One-off population census: A census is far too infrequent.
• DHS survey every 5 years: A DHS survey every 5 years is too infrequent.
• Sentinel hospital only: A single sentinel hospital misses most facilities.

Answer: About one. In a normal (Gaussian) distribution — symmetrical, bell-shaped, mean = median = mode — about 68% of values lie within ±1 SD, 95% within ±1.96 (≈ 2) SD and 99.7% within ±3 SD. Reference ranges usually cover the central 95% of healthy people, so 5% of healthy results fall outside (2.5% each tail). With multiple independent tests, the chance of at least one "abnormal" result rises (20 tests → ~64% chance of ≥ 1 abnormal; expected number ~1). Interpret mild abnormalities in context and avoid unnecessary test panels. Standard normal deviate (z) = (value − mean)/SD.

• None: Some abnormal results are expected by chance (about 5% each).
• About ten: About ten overestimates; 5% of 20 is about one.
• About five: Expected ~1 (5% of 20).

Answer: cVDPV2 — nOPV2 campaigns. OPV (live attenuated, Sabin) gives gut immunity and interrupts transmission but rarely causes vaccine-associated paralytic polio in recipients/contacts, and in communities with low coverage, excreted vaccine virus can circulate for months, mutate and regain neurovirulence → cVDPV outbreaks (mostly type 2). Since wild type 2 was eradicated, trivalent OPV was replaced by bivalent OPV (types 1 and 3) in 2016 ("the switch"), with IPV added for type 2 protection. cVDPV2 outbreaks are controlled with high-coverage campaigns using novel OPV2 (more genetically stable). IPV (inactivated) protects against paralysis but gives limited gut immunity, so doesn’t stop transmission alone. High coverage prevents cVDPV emergence.

• IPV-derived polio — stop IPV: IPV is inactivated and can’t revert.
• Wild type 2 — IPV catch-up: Wild type 2 is eradicated.
• OPV overdose — pause OPV: Under-immunisation, not over-vaccination, causes cVDPV.

Answer: Neurolathyrism. Lathyrism: chronic consumption of Lathyrus sativus (khesari/grass pea — a drought-resistant pulse) as a major part of the diet (> about one-third for 2–3 months) → neurotoxin β-N-oxalyl-L-α,β-diaminopropionic acid (β-ODAP, also called BOAA) damages upper motor neurones → irreversible spastic paraparesis (stages: no stick, one stick, two sticks, crawler), mainly young men. Prevention: vitamin C prophylaxis, removing toxin by steeping (soaking in hot water and discarding — "steeping method") or parboiling, banning/diluting sale, low-toxin varieties, food security. Epidemic dropsy — argemone oil adulteration of mustard oil (sanguinarine): oedema, heart failure, glaucoma.

• Epidemic dropsy: Epidemic dropsy causes oedema and glaucoma from contaminated mustard oil.
• Beriberi: Beriberi is due to thiamine deficiency.
• Konzo: Cassava — not khesari dal.

Answer: Small population shift prevents more cases. Rose’s strategies: high-risk strategy — identify and treat individuals at high risk (screening, drugs); appropriate for individuals, motivating, but limited population impact, temporary and doesn’t address causes. Population strategy — shift the whole distribution of a risk factor (salt reduction, tobacco taxes, trans-fat bans, built environment) — larger total impact because a large number of people at small risk give rise to more cases than a small number at high risk (prevention paradox: a measure bringing large benefits to the community offers little to each participating individual). Both are complementary. WHO "best buys" for NCDs include salt reduction, tobacco and alcohol taxation, and hypertension/diabetes management in primary care.

• Only high-risk matter: Most cases from moderate risk.
• Individual treatment prevents more cases: Treating high-risk individuals prevents fewer cases overall than shifting the whole population.
• Population measures don’t work: Population measures do work, as with salt reduction.

Answer: Schmutzdecke. Slow sand filter: supernatant raw water → sand bed → gravel → under-drainage. The ‘vital layer’ (Schmutzdecke) — a slimy biological layer of algae, bacteria and protozoa forming on the sand surface within days — removes 98–99% of bacteria and organic matter. Cleaned by scraping the top layer. Rapid sand (mechanical) filters need prior coagulation (alum) and back-washing; less effective bacteriologically, followed by chlorination.

• Coarse gravel base: The gravel only supports the sand.
• Under-drainage pipes: Drains only collect water.
• Supernatant water layer: The supernatant provides the head of water.

Answer: Mann–Whitney U test. Choosing tests: two independent groups — continuous normal: unpaired t-test; skewed/ordinal: Mann–Whitney U. Paired groups — normal: paired t-test; skewed: Wilcoxon signed-rank. > 2 groups — ANOVA (normal) or Kruskal–Wallis. Categorical proportions — chi-square (Fisher exact if small expected counts); paired categorical — McNemar. Correlation: Pearson (normal, linear), Spearman (ranked/skewed).

• Unpaired t-test: The t-test assumes normality.
• Chi-square test: Chi-square is for categorical data.
• Paired t-test: The groups are independent, not paired.

Answer: Prevalence < 1 per 10,000. WHO definition: elimination of leprosy as a public health problem = registered prevalence below 1 case per 10,000 population. Pakistan achieved this in 1996 (Marie Adelaide Leprosy Centre, Dr Ruth Pfau). Current WHO strategy targets zero transmission, zero disability, zero stigma; single-dose rifampicin post-exposure prophylaxis for contacts; MDT for 6 (PB) or 12 months (MB). Elimination (of transmission) vs eradication (worldwide zero — only smallpox).

• Zero new cases nationally: Zero new cases would be interruption of transmission.
• Incidence < 1 per 1000: Incidence is the wrong metric.
• No new disabilities in children: Child disability is a different target.

Answer: Radon gas. Radon: naturally occurring radioactive gas from uranium decay in soil and rock; accumulates indoors (basements, poorly ventilated homes) and in underground mines; alpha-emitting progeny irradiate bronchial epithelium → lung cancer (second leading cause after smoking; synergistic with smoking). Prevention: testing, ventilation, sealing foundations. Other occupational lung carcinogens: asbestos, arsenic, chromium, nickel, polycyclic aromatic hydrocarbons, silica.

• Carbon monoxide: CO is not carcinogenic.
• Ozone: Ozone is a respiratory irritant.
• Nitrogen dioxide: NO₂ is a respiratory irritant.

Answer: Mental Health Ordinance 2001. The Lunacy Act 1912 (colonial, custodial) was replaced by the Mental Health Ordinance 2001, which introduced informed consent, rights of patients, categories of admission (assessment, treatment, urgent/emergency), review by a mental health authority and safeguards against abuse. After the 18th Amendment (2010), health became a provincial subject: Sindh Mental Health Act 2013, Punjab (amended Ordinance 2014), KP Mental Health Act 2017 and others. Attempted suicide was decriminalised in 2022 (PPC section 325 repealed).

• Pakistan Penal Code 1860: The PPC is criminal law.
• PMDC Ordinance 1962: The PMDC Ordinance governs medical registration.
• Code of Criminal Procedure 1898: The CrPC governs criminal trials.