Infectious Diseases Practice Questions

Q: What is the most common cause of Hepatitis B in a 3-year-old child?

Perinatal transmission. **Explanation:** The most common cause of Hepatitis B Virus (HBV) infection in children worldwide, particularly in endemic regions, is **Perinatal (Vertical) Transmission**. This occurs from an HBsAg-positive mother to her newborn during delivery. In a 3-year-old child, the infection is almost always a result of this early exposure, as the risk of developing chronic HBV is inversely proportional to the age at which the infection is acquired (90% risk in neonates vs. <5% in adults). **Analysis of Options:** * **Perinatal Transmission (Correct):** This is the primary driver of the pediatric HBV burden. Transmission usually occurs during birth via contact with maternal blood and vaginal secretions. * **Pin prick (Incorrect):** While a risk for healthcare workers or through accidental needle sticks, it is a rare occurrence in the general pediatric population compared to vertical transmission. * **Saliva exchange (Incorrect):** Although HBV DNA can be detected in saliva, it is not considered an efficient or common route of transmission, especially in early childhood. * **Blood transfusion (Incorrect):** Due to mandatory and stringent screening of blood donors for HBsAg, transfusion-associated Hepatitis B is now extremely rare. **Clinical Pearls for NEET-PG:** * **Chronicity Risk:** The younger the age of infection, the higher the risk of chronicity. * **Immunoprophylaxis:** To prevent vertical transmission, infants born to HBsAg+ mothers should receive both the **HBV Vaccine** and **Hepatitis B Immunoglobulin (HBIG)** within 12 hours of birth. * **HBeAg Status:** If the mother is HBeAg positive, the risk of transmission to the baby is as high as 70-90%. * **Horizontal Transmission:** In older children, household contact (sharing toothbrushes or razors) is the second most common route in endemic areas.

Q: An unimmunized child presents with signs suggestive of diphtheria. Which type of diphtheria would suggest the worst prognosis?

Laryngeal. **Explanation:** The prognosis of diphtheria is primarily determined by the site of infection and the degree of toxin absorption. **Why Laryngeal Diphtheria is the worst prognosis:** Laryngeal involvement is considered the most dangerous form because it carries a high risk of **acute airway obstruction**. The characteristic "pseudomembrane" can detach or cause significant edema, leading to stridor, respiratory distress, and death by asphyxiation. Furthermore, laryngeal diphtheria is often a downward extension of pharyngeal infection, indicating a high toxin load and a greater risk of systemic complications like myocarditis and neuritis. **Analysis of Incorrect Options:** * **Tonsillopharyngeal:** This is the most common form. While it can lead to significant toxin absorption (causing "bull neck" lymphadenopathy), the immediate risk of sudden airway occlusion is lower than in the laryngeal form. * **Nasal:** This is generally the mildest form. It presents with serosanguinous discharge. Because the nasal mucosa is less vascularized for toxin absorption, systemic complications are rare. * **Cutaneous:** This presents as non-healing ulcers. It often acts as a reservoir for the bacteria but rarely leads to systemic toxicity or mortality. **Clinical Pearls for NEET-PG:** * **Agent:** *Corynebacterium diphtheriae* (Gram-positive, club-shaped, Chinese-letter pattern). * **Pathogenesis:** Mediated by an exotoxin that inhibits protein synthesis via **ADP-ribosylation of Elongation Factor-2 (EF-2)**. * **Diagnosis:** Culture on **Löffler's serum slope** or **Potassium Tellurite agar** (black colonies). * **Treatment:** Immediate administration of **Diphtheria Antitoxin (DAT)** is the priority, as it only neutralizes unbound toxin. Antibiotics (Penicillin or Erythromycin) are used to stop further toxin production and spread.

Q: Which of the following organisms causes pneumatocele in children in association with pneumonia?

Staphylococcus aureus. **Explanation:** **Staphylococcus aureus** is the most common cause of **pneumatoceles** (thin-walled, air-filled cysts within the lung parenchyma) in children. The underlying mechanism involves the production of toxins and enzymes (such as Panton-Valentine Leukocidin) that cause focal necrosis of the bronchial wall. This creates a "check-valve" mechanism where air enters the interstitial space during inspiration but becomes trapped during expiration, leading to the formation of these characteristic air-filled cavities. **Analysis of Options:** * **Streptococcus pneumoniae:** While it is the most common cause of community-acquired pneumonia in children, it typically presents with lobar consolidation. It rarely causes cavitation or pneumatoceles. * **Klebsiella pneumoniae:** This organism is known for causing "Friedlander’s pneumonia" with a "bulging fissure sign." While it can cause lung abscesses and necrosis (especially in neonates or immunocompromised adults), it is not the classic association for pneumatoceles in the pediatric population. * **Haemophilus influenzae:** Typically causes bronchopneumonia or pleural effusions/empyema, but it does not have the necrotizing tendency required to form pneumatoceles. **High-Yield Clinical Pearls for NEET-PG:** * **Staphylococcal Pneumonia:** Often follows a viral prodrome (like Influenza). It is characterized by rapid progression, empyema, pyopneumothorax, and pneumatoceles. * **Management:** Most pneumatoceles are asymptomatic and resolve spontaneously over weeks to months; surgical intervention is rarely required unless they cause tension pneumothorax or become infected. * **Radiology:** Look for the "Air-fluid level" within a thin-walled cyst on a chest X-ray in a child with severe respiratory distress.

Q: A baby is born to a mother with AIDS. Which of the following statements is true regarding the infant, except?

50% chance of acquiring AIDS. In vertical transmission of HIV, the risk of a baby acquiring the infection from an untreated mother is approximately **25–30%** (not 50%). With modern interventions, including Highly Active Antiretroviral Therapy (HAART) for the mother, elective cesarean section, and avoidance of breastfeeding, this risk can be reduced to **less than 1–2%**. Therefore, Option C is the incorrect statement (the "except" answer). **Explanation of Options:** * **Option A (Failure to Thrive):** This is a hallmark clinical feature of pediatric HIV. Affected infants often show poor weight gain and growth retardation due to chronic infections, malabsorption, and the hypermetabolic state of the disease. * **Option B (Infections):** HIV-infected infants are highly susceptible to opportunistic infections. Common presentations include recurrent bacterial infections, persistent oral candidiasis (thrush), and *Pneumocystis jirovecii* pneumonia (the most common opportunistic infection in the first year of life). * **Option D:** Since Option C is statistically inaccurate, "All of the above" cannot be the correct choice. **NEET-PG High-Yield Pearls:** 1. **Diagnosis:** In infants <18 months, **DNA-PCR** (detecting the virus) is the gold standard because maternal IgG antibodies cross the placenta and can persist for up to 18 months, making ELISA unreliable (false positives). 2. **Prophylaxis:** All HIV-exposed infants should receive **Cotrimoxazole prophylaxis** starting at 4–6 weeks of age to prevent *Pneumocystis* pneumonia until infection is ruled out. 3. **Feeding:** In India (NACO guidelines), exclusive breastfeeding is recommended for the first 6 months if replacement feeding is not "Acceptable, Feasible, Affordable, Sustainable, and Safe" (AFASS). Mixed feeding should be strictly avoided.

Q: An 18-month-old baby with poor feeding and fever for 3 days presents with lower limb weakness. On examination, the baby is lethargic, has abnormal movement of the lower limbs, and the anterior fontanelle is bulging. What is the diagnosis?

Meningitis. **Explanation:** The clinical presentation of fever, poor feeding, and lethargy in an 18-month-old is highly suggestive of an acute infection. The presence of a **bulging anterior fontanelle** is a hallmark sign of increased intracranial pressure (ICP), which, in the context of fever, strongly points toward **Acute Bacterial Meningitis**. The "abnormal movement" and lower limb weakness are likely manifestations of meningeal irritation or focal neurological deficits resulting from inflammation or vasculitis associated with the infection. **Why other options are incorrect:** * **Intracranial Hemorrhage:** While it causes a bulging fontanelle and lethargy, it usually presents with a sudden onset, often following trauma or in the context of a bleeding diathesis (e.g., Vitamin K deficiency), and typically lacks a high-grade fever. * **Pseudotumor Cerebri (Idiopathic Intracranial Hypertension):** This condition presents with signs of raised ICP (headache, papilledema) but is rare in infants and is characteristically associated with a lack of fever and normal CSF composition. * **Cerebral Palsy:** This is a non-progressive permanent disorder of posture and movement resulting from an insult to the developing brain (often prenatal or perinatal). It does not present acutely with fever and a bulging fontanelle. **High-Yield Pearls for NEET-PG:** * **Most common organisms:** *Streptococcus pneumoniae* is the most common cause of bacterial meningitis in children >1 month of age. * **Clinical Signs:** In infants, classic signs like Kernig’s and Brudzinski’s are often absent. Look for non-specific signs: bulging fontanelle, high-pitched cry, and paradoxical irritability. * **Diagnosis:** Lumbar puncture is the gold standard (unless signs of herniation are present). * **Treatment:** Empiric therapy usually involves 3rd generation cephalosporins (Ceftriaxone/Cefotaxime) plus Vancomycin.

Q: The teeth abnormality shown in the image is seen in which of the following diseases?

Congenital Syphilis. ***Congenital Syphilis*** - The dental abnormalities shown are likely **Hutchinson's teeth** (notched, peg-shaped incisors) and **mulberry molars** (dome-shaped first molars), pathognomonic of congenital syphilis. - These findings are part of **Hutchinson's triad** which includes interstitial keratitis, eighth nerve deafness, and Hutchinson's teeth. *Cleidocranial dysostosis* - Dental abnormalities include **delayed tooth eruption**, **supernumerary teeth**, and **impacted permanent teeth**. - Associated with **absent or hypoplastic clavicles** and **delayed fontanelle closure**, not the specific tooth morphology seen in the image. *Congenital Rubella* - Dental manifestations are minimal and include **delayed tooth eruption** and **enamel defects**. - Primary features are **cataracts**, **cardiac defects**, and **sensorineural hearing loss**, not characteristic dental malformations. *Congenital hypothyroidism* - Causes **delayed tooth eruption** and **prolonged retention of deciduous teeth**. - Associated with **growth retardation** and **mental retardation**, but does not cause the specific morphological tooth changes shown.

Q: Hutchinson teeth are a characteristic feature of which condition?

Late congenital syphilis. **Explanation:** **Hutchinson teeth** are a classic dental abnormality characterized by widely spaced, peg-shaped permanent incisors with a central notch on the cutting edge. This occurs due to the direct invasion of the developing tooth germs by *Treponema pallidum*. 1. **Why B is correct:** Hutchinson teeth are a component of the **Hutchinson’s Triad**, which is diagnostic of **Late Congenital Syphilis** (manifesting after 2 years of age). The triad includes: * Hutchinson teeth (Incisors) * Interstitial keratitis (leading to blindness) * Eighth nerve deafness (Sensorineural hearing loss) 2. **Why other options are incorrect:** * **A. Early congenital syphilis:** Presents before 2 years of age with features like snuffles (rhinitis), skin rashes (pemphigus syphiliticus), and hepatosplenomegaly. Dental changes only appear when permanent teeth erupt later in life. * **C. Rickets:** Characterized by delayed eruption of teeth and enamel hypoplasia, but not the specific notched, peg-shaped morphology. * **D. Scurvy:** Primarily affects the gums (swollen, bleeding) and leads to loose teeth due to defective collagen synthesis, but does not cause Hutchinson teeth. **High-Yield Clinical Pearls for NEET-PG:** * **Mulberry Molars:** Another dental sign of late congenital syphilis involving the first lower molars, which have multiple tiny globules of enamel instead of cusps. * **Saber Shins:** Anterior bowing of the tibia seen in late syphilis. * **Clutton’s Joints:** Painless, symmetrical swelling of the knees (hydrarthrosis). * **Higouménakis sign:** Unilateral thickening of the inner third of the clavicle.

Q: A 12-year-old child presents with fever and cervical lymphadenopathy for the last 36 hours. Oral examination shows a grey membrane on the right tonsil. What is the primary treatment?

20,000-40,000 units IM equine-derived antitoxin. **Explanation:** The clinical presentation of fever, cervical lymphadenopathy ("bull neck" appearance), and a greyish adherent membrane on the tonsils is classic for **Faucial Diphtheria** caused by *Corynebacterium diphtheriae*. **Why Option B is Correct:** The primary and most urgent treatment for diphtheria is the administration of **Diphtheria Antitoxin (DAT)**. Since the exotoxin causes irreversible tissue damage and myocarditis, the antitoxin must be given immediately based on clinical suspicion without waiting for culture results. The dosage is determined by the site and severity of the disease: * **Tonsillar/Pharyngeal (limited):** 20,000–40,000 units. * **Nasopharyngeal:** 40,000–60,000 units. * **Severe/Late presentation (>48 hours) or Systemic:** 80,000–120,000 units. As this patient has localized tonsillar involvement and presented within 36 hours, 20,000–40,000 units is the appropriate dose. **Why Other Options are Incorrect:** * **Option A & D:** Antibiotics (Penicillin or Erythromycin) are **secondary** treatments. They serve to stop further toxin production and prevent the spread of the organism to others, but they do not neutralize the toxin already circulating in the body. * **Option C:** This higher dose is reserved for extensive disease, laryngeal involvement, or patients presenting late (usually >3 days of symptoms). **High-Yield Clinical Pearls for NEET-PG:** 1. **Schick Test:** Used to determine the immune status of an individual (not for diagnosis). 2. **Culture Media:** Löffler’s serum slope (rapid growth) and Potassium Tellurite agar (black colonies). 3. **Microscopy:** Albert stain shows metachromatic granules (Babes-Ernst granules) in a "Chinese letter" pattern. 4. **Most Common Complication:** Myocarditis (usually occurs in the 2nd week). 5. **Neurological Complication:** Palatal palsy is the most common early neurological sign.

Question 1

What is the most common cause of Hepatitis B in a 3-year-old child?

Accepted Answer

Perinatal transmission

Answer

Pin prick

Answer

Saliva exchange

Answer

Blood transfusion

Question 2

An unimmunized child presents with signs suggestive of diphtheria. Which type of diphtheria would suggest the worst prognosis?

Accepted Answer

Laryngeal

Answer

Tonsillopharyngeal

Answer

Nasal

Answer

Cutaneous

Question 3

Which of the following organisms causes pneumatocele in children in association with pneumonia?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Answer

Klebsiella

Answer

Haemophilus influenzae

Question 4

A baby is born to a mother with AIDS. Which of the following statements is true regarding the infant, except?

Accepted Answer

50% chance of acquiring AIDS

Answer

Failure to thrive

Answer

Infections

Answer

All of the above are true

Question 5

A 4-month-old infant presents with failure to thrive, progressive muscular weakness, and poor head control. The mother states that she feeds the baby soy-based formula sweetened with honey. Which organism is most likely responsible for these symptoms?

Accepted Answer

Clostridium botulinum

Answer

Clostridium difficile

Answer

Clostridium perfringens

Answer

Clostridium tetani

Question 6

An 18-month-old baby with poor feeding and fever for 3 days presents with lower limb weakness. On examination, the baby is lethargic, has abnormal movement of the lower limbs, and the anterior fontanelle is bulging. What is the diagnosis?

Accepted Answer

Meningitis

Answer

Intracranial hemorrhage

Answer

Pseudotumor cerebri

Answer

Cerebral palsy

Question 7

The teeth abnormality shown in the image is seen in which of the following diseases?

Accepted Answer

Congenital Syphilis

Answer

Cleidocranial dysostosis

Answer

Congenital Rubella

Answer

Congenital hypothyroidism

Question 8

Which of the following is true about erythema infectiosum?

Accepted Answer

Defervescence occurs before the rash appears

Answer

Slapped cheek appearance is seen

Answer

It is caused by parvovirus

Answer

The rash appears on the head and neck

Question 9

Hutchinson teeth are a characteristic feature of which condition?

Accepted Answer

Late congenital syphilis

Answer

Early congenital spirochetal infection

Answer

Rickets

Answer

Scurvy

Question 10

A 12-year-old child presents with fever and cervical lymphadenopathy for the last 36 hours. Oral examination shows a grey membrane on the right tonsil. What is the primary treatment?

Accepted Answer

20,000-40,000 units IM equine-derived antitoxin

Answer

Penicillin IV for 10 days

Answer

80,000-120,000 units IM equine-derived antitoxin

Answer

Erythromycin for 2 weeks

Infectious Diseases — MCQs

Infectious Diseases — MCQs

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