A 5-month old infant is brought in for routine visit. What is the probable cause for the following findings?
**Core Concept**
The infant's findings are indicative of a congenital condition affecting the metabolism of a crucial amino acid. This condition is caused by a deficiency in a key enzyme responsible for breaking down this amino acid, leading to its accumulation and subsequent toxicity.
**Why the Correct Answer is Right**
The probable cause is a deficiency in the enzyme phenylalanine hydroxylase, which is necessary for converting phenylalanine into tyrosine. This enzyme deficiency leads to the accumulation of phenylalanine in the body, causing a condition known as phenylketonuria (PKU). PKU is a genetic disorder that can cause intellectual disability, seizures, and other neurological problems if left untreated. The symptoms in the infant, such as developmental delay and seizures, are consistent with PKU.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct enzyme deficiency. While other enzyme deficiencies can cause metabolic disorders, they do not match the symptoms described.
**Option B:** This option is incorrect because it refers to a different amino acid and enzyme. While it is a metabolic disorder, it does not match the specific symptoms described.
**Option C:** This option is incorrect because it refers to a different condition altogether, which is not related to the symptoms described.
**Clinical Pearl / High-Yield Fact**
It is essential to screen newborns for PKU through a heel prick blood test to detect this condition early, as prompt treatment can prevent intellectual disability and other complications.
**Correct Answer: B. Phenylketonuria.**