**Core Concept**
Childhood poliomyelitis, caused by the poliovirus, leads to a range of neurological symptoms due to viral invasion of the anterior horn cells in the spinal cord. The resulting acute flaccid paralysis is a hallmark of the disease, affecting motor neurons responsible for muscle contraction.

**Why the Correct Answer is Right**
The anterior horn cells in the spinal cord are the primary target of the poliovirus, leading to their destruction and subsequent paralysis of the affected muscles. The muscles of the lower limbs, particularly those involved in ambulation, are most frequently affected due to the weight-bearing demands placed on them. This is why muscles like the quadriceps, hamstrings, and gastrocnemius are commonly implicated.

**Why Each Wrong Option is Incorrect**
**Option A:** The intercostal muscles are not typically affected in childhood poliomyelitis, as they are not as heavily involved in ambulation or other weight-bearing activities.
**Option B:** The diaphragm is a critical muscle for respiration, but it is not the most frequently affected in childhood poliomyelitis, as the poliovirus tends to target muscles involved in ambulation.
**Option C:** The muscles of the upper limbs, such as the biceps and triceps, can be affected in poliomyelitis, but they are less frequently affected than the muscles of the lower limbs.

**Clinical Pearl / High-Yield Fact**
In poliomyelitis, the degree of paralysis is often related to the severity of the illness and the extent of viral invasion of the anterior horn cells. Early recognition and treatment are crucial in preventing long-term sequelae, such as muscle atrophy and contractures.

**Correct Answer: D. Quadriceps.**