## **Core Concept**
The question presents a clinical scenario suggestive of **Duchenne Muscular Dystrophy (DMD)**, a genetic disorder characterized by progressive muscle degeneration and weakness. This condition primarily affects boys and is caused by an absence of **dystrophin**, a protein crucial for muscle function. The symptoms described, such as difficulty climbing stairs, swollen calves (pseudohypertrophy), and the use of Gowers' sign (using feet to stand up), are classic for DMD.

## **Why the Correct Answer is Right**
The clinical presentation strongly suggests DMD. The next diagnostic step would involve confirming the diagnosis through genetic testing, which can identify mutations in the **dystrophin gene**. This is crucial for confirming the diagnosis and for genetic counseling.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While electromyography (EMG) and muscle biopsy are diagnostic tools for muscular dystrophies, they are not typically the next step after clinical evaluation. Genetic testing has become the preferred initial diagnostic approach due to its specificity and the information it provides about the mutation.
- **Option B:** Although assessing **creatine kinase (CK) levels** can support the diagnosis by showing significant elevation, which is indicative of muscle damage, it is not specific for DMD and does not provide a definitive diagnosis.
- **Option C:** This option is not provided, but typically, other diagnostic methods like EMG or muscle biopsy might be considered in the diagnostic pathway but are not usually the immediate next step after clinical suspicion.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is the early recognition of Gowers' sign and pseudohypertrophy of the calves, which are highly suggestive of DMD. Early diagnosis through **genetic testing** allows for early intervention, including **corticosteroid therapy**, which can slow disease progression, and planning for long-term care.

## **Correct Answer:** . **Genetic testing**