The recommended children's dosage of Keflex is 25 to 50 mg/kg/day in divided doses. The maximum daily safe dosage for a child weighing 42 lb is: Round to the nearest tenth.

(Number Only, do not enter mg, kg, or day with your answer)

The correct answer is choice C: Concrete operations stage.

Choice A rationale:

The preoperational stage, which occurs roughly between ages 2 and 7, is characterized by egocentrism, animism, and a lack of conservation. Children in this stage struggle with understanding that quantities can remain the same even if their appearance changes. For instance, they might think that pouring water from a tall glass into a short, wide glass somehow changes the amount of water. This stage does not demonstrate the mental understanding required for the examples given in the question.

Choice B rationale:

The intuitive thought stage, which typically spans from ages 4 to 7, is marked by increased symbolic thinking but still lacks the capacity for logical and systematic reasoning. Children in this stage might be able to count and perform basic arithmetic, but they struggle with more complex operations and understanding the underlying principles of mathematical operations. This stage is not where the ability to understand the given math operations is firmly established.

Choice C rationale:

The concrete operations stage, which encompasses ages 7 to 11, is characterized by the development of logical and systematic thinking. Children in this stage are capable of understanding basic mathematical principles and operations, such as conservation of quantity and simple arithmetic. They can mentally manipulate numbers and understand that reversing an operation will yield the original quantity. Therefore, this stage is when the ability to understand that 1 + 3 = 4 and 4 - 3 = 1 occurs.

Choice D rationale:

The formal operations stage, typically beginning around age 11, is characterized by abstract and hypothetical thinking. This is when adolescents and adults can engage in complex reasoning and consider multiple possibilities and outcomes. The given math operations are not the focus of this stage; rather, it pertains to more advanced cognitive abilities like hypothetical problem-solving and deductive reasoning.

The correct answer is choice A. Meconium ileus.

Choice A rationale:

Meconium ileus. This is the earliest recognizable clinical manifestation of cystic fibrosis (CF). Meconium ileus refers to the blockage of the small intestine by thick, sticky meconium, the earliest stool of a newborn. It occurs in the first 24-48 hours of life and can be a sign of underlying cystic fibrosis.

Choice B rationale:

Foul-smelling, frothy, greasy stools. Foul-smelling, frothy, greasy stools are characteristic of malabsorption, which can occur in cystic fibrosis. However, this manifestation typically appears after meconium ileus and is not the earliest recognizable clinical sign.

Choice C rationale:

History of poor intestinal absorption. While poor intestinal absorption is a common consequence of cystic fibrosis, it is not one of the earliest recognizable clinical manifestations. Meconium ileus takes precedence as the primary early sign.

Choice D rationale:

Recurrent pneumonia and lung infections. Recurrent pneumonia and lung infections are common in later stages of cystic fibrosis due to the buildup of thick mucus in the respiratory tract. However, they are not among the earliest recognizable clinical manifestations of the disease. Meconium ileus is the earliest indicator in this context.