Correct Answer: A

Rationale: Trisomy 21, also known as Down syndrome, is caused by nondisjunction, which is an error in cell division that leads to the presence of an extra chromosome 21. This additional chromosome alters the normal genetic makeup, resulting in the characteristics associated with Down syndrome. Therefore, the correct answer is nondisjunction (Choice A). Choices B, C, and D are incorrect because Trisomy 21 is not caused by X-linked recessive inheritance, genomic imprinting, or autosomal dominant inheritance. Understanding the specific genetic mechanism involved in Trisomy 21 is crucial for grasping the basis of this chromosomal disorder.

Correct Answer: D

Rationale: The correct answer is D: Severe coughing spells. Pertussis, also known as whooping cough, typically presents with severe coughing spells that can be followed by a characteristic 'whoop' sound. These coughing fits can be intense and prolonged, often causing the child to gasp for air between coughs. Option A, dry hacking cough, is a common symptom of other respiratory conditions like bronchitis. Option B, inspiratory stridor, is more commonly associated with conditions like croup. Option C, nasal congestion, is not a typical symptom of pertussis.

Correct Answer: D

Rationale: Recognizing signs of hypoglycemia is crucial for managing type 1 diabetes mellitus and preventing complications.

Correct Answer: B

Rationale: Edema is a hallmark clinical manifestation of nephrotic syndrome. In nephrotic syndrome, there is increased permeability of the glomerular filtration barrier, leading to protein loss in the urine (proteinuria). The decrease in serum protein levels results in a reduced oncotic pressure, leading to fluid shifting from the intravascular space into the interstitial spaces, causing edema. Jaundice (choice A) is not typically associated with nephrotic syndrome. Hypertension (choice C) is more commonly seen in conditions like nephritic syndrome. Polyuria (choice D) is excessive urination and is not a prominent feature of nephrotic syndrome.

Correct Answer: B

Rationale: The correct answer is B. Live vaccines like MMR should not be given to immunosuppressed infants because their weakened immune systems may not handle the vaccine safely. Choice A is incorrect as delaying the MMR vaccine until the infant reaches 2 years of age does not address the issue of immunosuppression. Choice C is incorrect because administering live vaccines to an immunosuppressed individual could lead to serious complications. Choice D is incorrect as immediate action is required to prevent potential harm to the immunosuppressed infant.